Methods for Embedding Cell-Free Protein Synthesis Reactions in Macro-Scale Hydrogels

Summary

Here, we present two protocols for embedding cell-free protein synthesis reactions in macro-scale hydrogel matrices without the need for an external liquid phase.

Abstract

Synthetic gene networks provide a platform for scientists and engineers to design and build novel systems with functionality encoded at a genetic level. While the dominant paradigm for the deployment of gene networks is within a cellular chassis, synthetic gene networks may also be deployed in cell-free environments. Promising applications of cell-free gene networks include biosensors, as these devices have been demonstrated against biotic (Ebola, Zika, and SARS-CoV-2 viruses) and abiotic (heavy metals, sulfides, pesticides, and other organic contaminants) targets. Cell-free systems are typically deployed in liquid form within a reaction vessel. Being able to embed such reactions in a physical matrix, however, may facilitate their broader application in a wider set of environments. To this end, methods for embedding cell-free protein synthesis (CFPS) reactions in a variety of hydrogel matrices have been developed. One of the key properties of hydrogels conducive to this work is the high-water reconstitution capacity of hydrogel materials. Additionally, hydrogels possess physical and chemical characteristics that are functionally beneficial. Hydrogels can be freeze-dried for storage and rehydrated for use later. Two step-by-step protocols for the inclusion and assay of CFPS reactions in hydrogels are presented. First, a CFPS system can be incorporated into a hydrogel via rehydration with a cell lysate. The system within the hydrogel can then be induced or expressed constitutively for complete protein expression through the hydrogel. Second, cell lysate can be introduced to a hydrogel at the point of polymerization, and the entire system can be freeze-dried and rehydrated at a later point with an aqueous solution containing the inducer for the expression system encoded within the hydrogel. These methods have the potential to allow for cell-free gene networks that confer sensory capabilities to hydrogel materials, with the potential for deployment beyond the laboratory.

Introduction

Synthetic biology integrates diverse engineering disciplines to design and engineer biologically based parts, devices, and systems that can perform functions that are not found in nature. Most synthetic biology approaches are still bound to living cells. By contrast, cell-free synthetic biology systems facilitate unprecedented levels of control and freedom in design, enabling increased flexibility and a shortened time for engineering biological systems while eliminating many of the constraints of traditional cell-based gene expression methods^1,2,3. CFPS is being used in an in....

Protocol

1. Cell lysate buffer and media preparation

1. Preparation of 2x YT+P agar and medium
   1. Prepare 2x YT+P agar by measuring out 16 g/L tryptone, 10 g/L yeast extract, 5 g/L NaCl, 40 mL/L 1 M K₂HPO₄, 22 mL/L 1 M KH₂PO₄, and 15 g/L agar. For the 2x YT+P broth, follow the previous composition but omit the agar.
   2. Sterilize by autoclaving the 2x YT+P.
2. Preparation of the S30A buffer
   1. Prepare the S30A buffer with 5.88 g/L Mg-glutamate, 12.195 g/L K-glutamate, and 25 mL/L 2 M Tris, adjusted to pH 7.7 with acetic acid.
   2. Store the S30A buffer at 4....

Results

This protocol details two methods for embedding CFPS reactions into hydrogel matrices, with Figure 1 presenting a schematic overview of the two approaches. Both methods are amenable to freeze-drying and long-term storage. Method A is the most utilized methodology for two reasons. First, it has been shown to be the most applicable method for working with a range of hydrogel materials¹¹. Second, Method A allows for the parallel testing of genetic constructs. Method B is.......

Discussion

Outlined here are two protocols for the incorporation of E. coli cell lysate-based CFPS reactions into agarose hydrogels. These methods allow for simultaneous gene expression throughout the material. The protocol can be adapted for other CFPS systems and has been successfully conducted with commercially available CFPS kits in addition to the laboratory-prepared cell lysates detailed here. Importantly, the protocol allows for gene expression in the absence of an external liquid phase. This means that the system i.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Material
3-PGA	Santa Cruz Biotechnology	sc-214793B
Acetic Acid	Sigma-Aldrich	A6283
Agar	Thermo Fisher Scientific	A10752.22
Agarose	Severn Biotech	30-15-50
Amino Acid Sampler Kit	VWR	BTRABR1401801
ATP	Sigma-Aldrich	A8937-1G
cAMP	Sigma-Aldrich	A9501-1G
Coenzyme A (CoA)	Sigma-Aldrich	C4282-100MG
CTP	Alfa Aesar	J14121.MC
DTT	Thermo Fisher Scientific	R0862
Folinic Acid	Sigma-Aldrich	F7878-100MG
GTP	Carbosynth	NG01208
HEPES	Sigma-Aldrich	H4034-25G
K-glutamate	Sigma-Aldrich	G1149-100G
Lysozyme	Sigma-Aldrich	L6876-1G
Mg-glutamate	Sigma-Aldrich	49605-250G
NAD	Sigma-Aldrich	N6522-250MG
PEG-8000	Promega	V3011
Potassium Hydroxide (KOH)	Sigma-Aldrich	757551-5G
Potassium Phosphate Dibasic (K2HPO4)	Sigma-Aldrich	P3786-500G
Potassium Phosphate Monobasic (KH2PO4)	Sigma-Aldrich	RDD037-500G
Protease Inhibitor cocktail	Sigma-Aldrich	P2714-1BTL
Qubit Protein concentration kit	Thermo Fisher Scientific	A50668
Rossetta 2 DE 3 E.coli	Sigma-Aldrich	71397-3
Sodium Chloride (NaCl)	Sigma-Aldrich	S9888-500G
Spermidine	Sigma-Aldrich	85558-1G
Tryptone	Thermo Fisher Scientific	211705
Tris	Sigma-Aldrich	GE17-1321-01
tRNA	Sigma-Aldrich	10109541001
UTP	Alfa Aesar	J23160.MC
Yeast Extract	Sigma-Aldrich	Y1625-1KG
Equipment
1.5 mL microcentrifuge tubes	Sigma-Aldrich	HS4323-500EA
10K MWCO dialysis cassettes	Thermo Fisher Scientific	66381
15 mL centrifuge tube	Sarstedt	62.554.502
50 mL centrifuge bottles	Sarstedt	62.547.254
500 mL centrifuge bottles	Thermo Fisher Scientific	3120-9500
Alpha 1-2 LD Plus freeze-dryer	Christ	part no. 101521, 101522, 101527
Benchtop Centrifuge	Thermo Fisher Scientific	H-X3R
Black 384 well microtitre plates	Fischer Scientific	66
Cuvettes	Thermo Fisher Scientific	222S
Elga Purelab Chorus	Elga	#####
Eppendorf Microcentrifuge 5425R	Eppendorf	EP00532
High Speed Centrifuge	Beckman Coulter	B34183
JMP license	SAS Institute	15
Magnetic Stirrer	Fischer Scientific	15353518
Parafilm	Amcor	PM-966
Photospectrometer (Biophotometer)	Eppendorf	16713
Pipettes and tips	Gilson	#####
Precision Balance	Sartorius	16384738
Qubit 2.0 Fluorometer	Thermo Fisher Scientific	Q32866
Shaking Incubator	Thermo Fisher Scientific	SHKE8000
Sonic Dismembrator (Sonicator)	Thermo Fisher Scientific	12893543
Static Incubator	Sanyo	MIR-162
Syringe and needles	Thermo Fisher Scientific	66490
Thermo max Q8000 (Shaking Incubator)	Thermo Fisher Scientific	SHKE8000
Varioskan Lux platereader	Thermo Fisher Scientific	VLBL00GD1
Vortex Genie 2	Cole-parmer	OU-04724-05
VWR PHenomenal pH 1100 L, ph/mv/°c meter	VWR	662-1657