Fabrication and Characterization of Colorectal Cancer Organoids from SW1222 Cell Line in Ultrashort Self-Assembling Peptide Matrix

Summary

This protocol aims to evaluate biofunctional self-assembling peptides for cell adhesion, organoid morphology, and gene expression by immunostaining. We will use a colorectal cancer cell line to provide a cost-effective way of obtaining organoids for intensive testing.

Abstract

Ultrashort self-assembling peptides (SAPs) can spontaneously form nanofibers that resemble the extracellular matrix. These fibers allow the formation of hydrogels that are biocompatible, biodegradable, and non-immunogenic. We have previously proven that SAPs, when biofunctionalized with protein-derived motifs, can mimic the extracellular matrix characteristics that support colorectal organoid formation. These biofunctional peptide hydrogels retain the original parent peptide’s mechanical properties, tunability, and printability while incorporating cues that allow cell-matrix interactions to increase cell adhesion. This paper presents the protocols needed to evaluate and characterize the effects of various biofunctional peptide hydrogels on cell adhesion and lumen formation using an adenocarcinoma cancer cell line able to form colorectal cancer organoids cost-effectively. These protocols will help evaluate biofunctional peptide hydrogel effects on cell adhesion and luminal formation using immunostaining and fluorescence image analysis. The cell line used in this study has been previously utilized for generating organoids in animal-derived matrices.

Introduction

In recent years, self-assembling peptides (SAPs) have emerged as promising biomaterials for tissue engineering applications. SAPs possess unique properties, including spontaneous formation of nanofibers, biocompatibility, biodegradability, and non-immunogenicity, making them attractive candidates for scaffold development¹. SAPs have been previously used together with various types of cells, and notably, reported ultrashort SAPs have facilitated the encapsulation of stem cells while upholding their pluripotency over prolonged periods encompassing more than 30 passages and with minimal incidence of chromosomal aberrations^2,3,4. Hence, adapting SAPs for their use in organoid culture constituted a rational subsequent step.

Organoids are complex three-dimensional structures that arise from single pluripotent cells. These structures give rise to various cell types, which then self-organize to replicate the developmental processes of embryonic and tissue growth in vitro⁵. This innovative framework has evolved into a formidable tool for in vitro investigations, efficiently conserving the genetic, phenotypic, and behavioral attributes characteristic of in vivo organs⁶. Nevertheless, a principal impediment in the bench-to-bedside transition of organoid-based findings is the need for reproducibility⁷. This variation in results is primarily ascribed to the difficulty in fully differentiating human pluripotent stem cells into specialized cell lineages, compounded by the absence of authentic tissue architecture and the inherent complexity encountered in organoid models. Given the rise in popularity of stem-cell- and organoid-based studies⁸, there has been an escalated demand for biomaterials with dynamic mechanical properties and integrin-like adhesion sites or scaffolds with controlled degradability^7,9. These attributes can be effectively tuned through the strategic utilization of biofunctionalized SAPs.

SAPs are short sequences of amino acids with the inherent ability to spontaneously organize into well-defined structures¹⁰. These peptides often contain alternating hydrophobic and hydrophilic residues, which drive their self-assembly through non-covalent interactions, including hydrogen bonding, electrostatic interactions, and hydrophobic effects^11,12. The self-assembly process of these peptides is primarily driven by the need to minimize the system's free energy. When in an aqueous environment, the hydrophobic residues tend to cluster together to minimize their exposure to water, while the hydrophilic residues interact with the surrounding water molecules. This phenomenon leads to the formation of various nanostructures. In this case, ultrashort self-assembling peptides form nanofibers with characteristics that depend on the peptide sequence and environmental conditions^{2,12,13,14,15,16}. These peptides adopt a β-turn conformation, where individual peptide strands align parallel or antiparallel to each other, stabilized by hydrogen bonds (Figure 1). The presence of positive ions accelerates the self-assembling processes that lead to nanofiber formation.

Peptide nanofibers have been identified as possessing an innate ability to entrap significant volumes of water. This characteristic facilitates the generation of a hydrogel, which subsequently manifests as a biocompatible and biodegradable three-dimensional space conducive to cellular proliferation and growth. These self-assembling peptide nanofibers intricately emulate the topographical features inherent to the natural extracellular matrix (ECM) environment¹. This resemblance endows cells with an environment that imitates their physiological habitat, further promoting optimal cellular activities¹⁷. Furthermore, the versatility inherent to these peptides permits a considerable degree of tunability⁴. Such adaptability enables changes to the matrix's properties, such as stiffness, gelation kinetics, and porosity. These adaptations are achieved through modifications to the peptide sequence. Consequently, self-assembling peptides (SAPs) have emerged as pivotal components in contemporary biomaterial science, extensively used as scaffolds for tissue regeneration and cellular culture^13,17.

One of the critical advantages of self-assembling peptides is their ability to be easily synthesized and modified at the molecular level. This advantage allows for the incorporation of specific functional groups or bioactive motifs into the peptide sequence, enabling the design of peptides with tailored properties and functionalities^18,19,20 (Figure 1B). For example, biofunctionalized SAPs can be designed to mimic the ECM and promote differentiation using the RGD peptide^19,21. Peptide Ben-IKVAV has also been reported to significantly increase the expression of neuronal-specific markers due to its ligand-specific motiety²². These peptides can also be engineered to display bioactive molecules, such as integrin-binding peptides, to enhance cell survival²³. Finally, other biofunctionalized SAPs have been developed to promote angiogenesis by including the IKVAV and YIGSR motifs in their structures²⁴.

Biofunctionalized SAPs reported in an earlier publication show that self-assembling can be further modified by including the naïve peptide from which the biofunctionalized SAP was derived²⁵. These SAP mixtures can also provide a wide array of SAP formulations that vary in their biochemical activity and physicochemical properties. For instance, the amphiphilic peptide IIFK is an ultrashort SAP that can withstand biofunctionalization with various motifs, exemplified by the incorporation of the IKVAV motif (Figure 1B). Both peptides can form nanofibers, both alone and combined. Each formulation results in hydrogels with varying physical properties (Figure 2)

However, because of the extensive array of potential permutations and alternatives obtained from the biofunctionalization of SAPs, there is a need to provide a fast and cost-effective option for organoid testing. One candidate for such intensive and cost-effective organoid evaluation is the SW1222 cell line, derived from human colorectal adenocarcinoma^26,27. SW1222 cells possess characteristics that enable their aggregation into 3D structures resembling organoids, making them an ideal model for studying tissue development and regenerative medicine applications. SW1222 cells have been identified as capable of generating organoids owing to their intrinsic overexpression of the LGR5 gene²⁷. The creation of organoids from individual LGR5⁺ stem cells has been convincingly exhibited before, as well as the propensity of SW1222 cells to achieve morphological characteristics of a colorectal cancer organoid²⁸.

In this methods paper, we present detailed protocols for evaluating and characterizing the effects of various biofunctional peptides on cell adhesion and lumen formation using SW1222 cells (Figure 3). By providing step-by-step procedures and imaging analysis methods, we hope to offer valuable insights into the biofabrication of organoids and the evaluation of simplistic SAP matrices for organoid culture.

Protocol

1. Buffer and solution preparation NOTE: All stated concentrations are final concentrations. Add fetal bovine serum (FBS) and penicillin-streptomycin at a final concentration of 10% and 1%, respectively, to prepare complete Iscove's Modified Dulbecco's Medium (IMDM). Store in the dark at 4 °C for up to 1 month. Mix MgCl2 (3 mM), sucrose (300 mM), and Triton X-100 (0.5%) in phosphate buffer saline (PBS) to prepare the permeabilizati…

Representative Results

First, we evaluated the cells grown in a 24-well plate for 7 days using brightfield imaging. We identified small clusters of cells assembling into organoids during the week, as seen in Figure 4. A controlled scan method can follow the mobility of the cells and the organoids between different days. In general, we looked at the evolution of the morphology of the cells during the whole week. SW1222-derived organoids should have a round morphology and a light appearance. A darker appearance indi…

Discussion

The vast potential of SAPs in biomedical research is underscored by their malleability and adaptability through biofunctionalization. With such an extensive array of permutations, the challenge arises in efficiently testing and determining the most promising configurations for specific applications, especially in organoid research. A fast, cost-effective solution is paramount. The SW1222 cell line, derived from human colorectal adenocarcinoma, emerges as a pivotal candidate for such intensive evaluation. Here, we present…

Materials

1x PBS	Gibco	14190144
6-well plate, tissue culture treated	Corning	07-200-83
10x PBS, no calcium, no magnesium	Gibco	70011044
16% Formaldehyde (w/v), Methanol-free	Thermo Scientific	28906
24-well plate, tissue culture treated	Corning	09-761-146
96-well black plate, tissue culture treated	Corning	07-200-565
alamarBlue Cell Viability Reagent	Invitrogen	DAL1025
Anti-Ezrin antibody, rabbit monoclonal	Abcam	ab40839	Secondary used: anti-rabbit Dylight 633
Anti-pan Cadherin antibody, rabbit polyclonal	Abcam	ab16505	Secondary used: anti-rabbit Alexa 488
Anti-ZO1 tight junction antibody, goat polyclonal	Abcam	ab190085	Secondary used: anti-goat Alexa 488
BSA	Sigma-Aldrich	A9418
Cellpose 2.0	NA	NA	Obtained from https://github.com/MouseLand/cellpose
Confocal Laser Scanning Microscope with Airyscan	ZEISS	LSM 880
DAPI	Invitrogen	D1306
Donkey anti-Goat IgG (H+L) Secondary Antibody, Alexa Fluor 488	Invitrogen	A-11055
Glycine	Cytiva	GE17-1323-01
Goat anti-Rabbit IgG (H+L) Secondary Antibody, Alexa Fluor 488	Invitrogen	A-11008
Goat anti-Rabbit IgG (H+L) Secondary Antibody, DyLight 633	Invitrogen	35562
Heat Inactivated Fetal Bovine Serum (HI FBS)	Gibco	16140071
ImageJ 1.54f	NIH	NA
IMDM	Gibco	12440079
LIVE/DEAD Viability/Cytotoxicity Kit, for mammalian cells	Invitrogen	L3224
Magnesium Chloride, hexahydrate (MgCl2 6 H2O)	Sigma-Aldrich	M2393
Matrigel for Organoid Culture, phenol-red free	Corning	356255	Refered in the manuscript as Matrigel or basement membrane matrix.
Microscope, brightfield
Microscope, EVOS	Thermo Scientific	EVOS M7000
OriginPro 2023 (64-bit) 10.0.0.154	OriginLab Corp	NA
Penicillin-Streptomycin (10,000 U/mL)	Gibco	15140122
Peptide P (Ac,Ile,Ile,Phe,Lys,NH2)	Lab-made	NA	Can be custom-made by peptide manufacturers such as Bachem.
Peptide P1 (Ac, Ile, Ile, Phe, Lys, Gly, Gly, Gly, Arg, Gly, Asp, Ser, NH2)	Lab-made	NA	Can be custom-made by peptide manufacturers such as Bachem.
Peptide P2 (Ac, Ile,Ile,Phe,Lys,Gly,Gly,Gly,Ile,Lys,Val,Ala,Val,NH2)	Lab-made	NA	Can be custom-made by peptide manufacturers such as Bachem.
Rhodamine Phalloidin	Invitrogen	R415
Round Cover Slip, 10 mm diameter	VWR	631-0170
Scanning Electron Microscope	Thermo Fisher – FEI	TENEO VS
Sterile 30 μm strainer	Sysmex	04-004-2326
sucrose	Sigma-Aldrich	S1888
SW1222 cell line	ECACC	12022910
Triton x-100	Thermo Scientific	85111
Trypsin-EDTA 0.25%	Gibco	25200056
Tween 20	Sigma-Aldrich	P1379
UltraPure water	Invitrogen	10977015