We describe a simple protocol to develop mucociliary epithelial organoids from deep ectoderm cells isolated from Xenopus laevis embryos. The multipotent progenitors regenerate epithelial goblet cell precursors and allow live tracking of the initiation and progression of the cell transitions on the surface of organoids.
Mucociliary epithelium provides the first line of defense by removing foreign particles through the action of mucus production and cilia-mediated clearance. Many clinically relevant defects in the mucociliary epithelium are inferred as they occur deep within the body. Here, we introduce a tractable 3D model for mucociliary epithelium generated from multipotent progenitors that were microsurgically isolated from Xenopus laevis embryos. The mucociliary epithelial organoids are covered with newly generated epithelium from deep ectoderm cells and later decorated with distinct patterned multiciliated cells, secretory cells, and mucus-producing goblet cells that are indistinguishable from the native epidermis within 24 h. The full sequences of dynamic cell transitions from mesenchymal to epithelial that emerge on the apical surface of organoids can be tracked by high-resolution live imaging. These in vitro cultured, self-organizing mucociliary epithelial organoids offer distinct advantages in studying the biology of mucociliary epithelium with high-efficiency in generation, defined culture conditions, control over number and size, and direct access for live imaging during the regeneration of the differentiated epithelium.
Injury, infection, and disease of mucociliary epithelium are associated with impaired production and clearance of mucus which is often found in pulmonary disorders such as chronic obstructive pulmonary disease, asthma, cystic fibrosis, bronchiectasis, and primary ciliary dyskinesia1,2,3,4. A recent advance in organoid technology, for instance, the basal cell derived lung organoid called tracheosphere that recapitulates the regeneration of mucociliary epithelium arise as a promising model with therapeutic potential1,5,6. However, its use is currently limited, in part because of lack of the defined culture conditions and low efficiency in organoid productions. Mucociliary epithelium in the human airway and frog epidermis are remarkably similar in tissue morphology, cellular composition, and its function7,8,9,10,11,12. In both organisms, mucociliary epithelium provides first-line defense by secreting mucus and antimicrobial substances and clears harmful particles and pathogens through the synchronized action of cilia.
Here, we describe a simple protocol to generate mucociliary epithelial organoids using the multipotent progenitors of Xenopus laevis embryos13,14. Previously, we reported14 that in the absence of exogenous growth factors and the extracellular matrix, the microsurgically isolated deep cells from the early gastrula stage ectoderm spontaneously assemble into aggregates, regenerate epithelium on its surface, and mature into mucociliary epithelium by intercalating multiciliated and other accessory cells within 24 h. In addition to the rapid development, this protocol offers a distinct opportunity for directly accessing the transitions of multipotent deep ectoderm cells into epithelial goblet cell progenitors that recapitulate the regeneration steps of a disrupted epithelium14 which are not available from intact embryos and ectoderm (also known as the animal cap)15,16,17. The number and size of the organoids produced are scalable with high efficiency by controlling the starting materials from Xenopus embryos. Organoids in floating culture can be easily sorted and transferred at the desired stage for further analyses, including high-resolution imaging, mechanical testing, drug treatment, and genetic characterization14. This spontaneous, tissue mechanics-driven regeneration of the epithelium on the surface of embryonic cell aggregates results in mucociliary epithelial organoids and provide a novel three-dimensional (3D) model to study the biology of the mucociliary epithelium.
Animal use and experimental protocols were approved by the institutional animal care and use committee (IACUC) of the Institute for Basic Science (IBS 18-01) and Korea Advanced Institute of Science and Technology (KA2017-22).
1. Embryos
2. Preparation of microsurgical tools, solutions, and culture vessels
3. Isolation of deep ectoderm cells
4. Generation of mucociliary epithelial organoids
5. (Optional) High-resolution live imaging of developing organoids
6. (Optional) Imaging developing organoids by fixation and immunostaining
This standardized protocol generates a mucociliary epithelial organoid from multipotent progenitors isolated from the early gastrula stage X. laevis embryos within 24 h of cultivation14. Collected deep ectoderm cells self-assemble to form an aggregate in a non-adhesive PCR tube and undergo surface epithelialization and goblet cell differentiation. The newly epithelialized surface of aggregates provides a substrate similar to the native epithelium found in vivo for intercalating inner cells (e.g., multiciliated and other accessory cells) and develops to form mucociliary epithelial organoids (Figure 1A,B). Within 24 h after aggregation, self-organized mucociliary epithelial organoids regenerate a mature epidermis that is indistinguishable from the epidermis of a tadpole. The organoids comprise fully differentiated epithelium (keratin), mucus-secreting goblet cells (ITLN), multiciliated cells (acetylated tubulin), and small secretory cells (peanut agglutinin, PNA) (Figure 1C).
In addition to confirming the development of different cell types with immunostaining, the dynamics of organoid development can be followed by live imaging (Figure 2A). To examine the epithelialization that emerges at the early stage of organoid formation (Figure 1B), we labeled embryos by expressing fluorescently tagged tight junction proteins (ZO-1-RFP) and membrane localizing proteins (mem-GFP). With dual-labeling, the sequential steps of ZO-1-positive tight junction formation can be marked and quantitatively analyzed during epithelialization (Figure 2). For example, for cells (Figure 2B, green-colored) at different stages of epithelialization (at 0 min), some regions of cell-cell adhesion have scattered puncta of ZO-1 (Figure 2B, green arrows). In contrast, other areas have fully assembled, contiguous ZO-1 expression (Figure 2B, yellow arrows). Over time, the puncta coalesce and connect to form contiguous tight junctions (Figure 2B, green arrows), and contiguous tight junctions maintain their morphology even during cell division (Figure 2B, yellow arrows). As the tight junctions mature, cells dynamically move in and out of the surface along the apical planes of the organoids (Figure 2C,D). Furthermore, by tracking cells spatiotemporally on the surface of differentiating organoids (Figure 2B, color-coded cells), multi-scale analysis is possible, ranging from individual puncta to contiguous tight junctions, cell-cell boundaries, and subsets of cell populations within organoids.
Figure 1: Generation of mucociliary epithelial organoids.
(A) A schematic showing the protocol to assemble deep ectoderm aggregates from X. laevis embryos. (B) A schematic for a model of mucociliary epithelial organoid formation originating from multipotent deep ectoderm cells (cross-sectional view). Surface-positioned cells transit into epithelial cells and differentiate into goblet cells. Differentiating ciliated cells, secretory cells, and ionocytes radially intercalate into the surface and regenerate a mature epidermis. (C) Maximum z-projection of mucociliary epithelium immunostained for ITLN (mucus-producing goblet cells), acetylated tubulin (ciliated cells), PNA (small secretory cells), and keratin (epithelial cells) in organoids at 24 hpa (upper panel) and tadpole epidermis (lower panel). Scale bar = 30 μm. Please click here to view a larger version of this figure.
Figure 2: Live imaging of developing organoids.
(A) A schematic of the imaging chamber for live organoids (not to scale). (B) Time-lapse sequences of confocal stacks collected from deep ectoderm cell aggregates expressing ZO-1-RFP and mem-GFP from 2.5 hpa. Scale bar = 20 μm. Cells are pseudo-colored for tracking over time. Green-colored cell have different cell adhesion statuses, including one progressively developing ZO-1 positive adhesion (green arrows) and one maintaining contiguous ZO-1 positive adhesion (yellow arrows) over time. (C, D) Time-lapse confocal images of ZO-1-RFP-expressing deep ectoderm cell aggregates show the radially intercalating cells moving to the surface (C, yellow star) and moving inside the aggregates (D, blue star). Scale bars = 10 μm. Please click here to view a larger version of this figure.
Mucociliary epithelial organoids generated from deep ectoderm cells of X. laevis embryo are a powerful tool to study the epithelialization and differentiation of multipotent progenitors in vitro. In contrast to the widely adopted animal cap assay16 used for in vitro organogenesis13 and the development of mucociliary epithelia15,17,22 that utilize the intact ectoderm, the deep ectoderm-derived organoids presented in this protocol offer a distinct opportunity to monitor the tissue mechanics-driven regeneration phases of the surface epithelium14. At around 2 hpa, the newly generated ZO-1 positive epithelial cells (Figure 2) begin to appear on the apical surface of organoids and increase their population to cover the entire organoid as the tissue solidify or reduces the compliance14. The regeneration of the epithelium and subsequent lineage specifications for mucus-producing goblet cells proceed spontaneously in a chemically defined culture media within a day. These rapidly developing mucociliary epithelial organoids provide a platform to examine dynamic cell behaviors in real-time, in high-resolution, during progressive steps of epithelial regeneration. They also enable investigation of fundamental questions that arise during mucociliary epithelium development, homeostasis, and associated diseases2,9,23. In particular, the mechanical sensitivity of the deep progenitor cells during the transition to epithelial goblet cell precursors identified in the organoids14 may serve to link respiratory diseases associated with abnormal basal differentiation where mucus-secreting goblet cells are over- or under-produced23.
While this protocol offers a simple approach to generate these organoids, there are several critical steps for success in experiments. To prevent contamination of superficial epithelial cells during the isolation of deep ectoderm cells from the animal cap, one should monitor the animal cap placed in calcium- and magnesium-free DFA under a stereoscope and detect the right time to initiate the separation of the dark-pigmented superficial layer of the animal cap. If the tissue is kept in calcium- and magnesium-free DFA for too long, the entire tissue will dissociate and distinguishing between deep and superficial cells would then be impossible for deep ectoderm aggregates. To confirm the absence of superficial cells in deep ectoderm aggregates, we recommend fluorescently labeling the apical surface of the embryo with NHS-rhodamine (step 1.414) prior to microsurgery; this would allow for easy identification of surface cells if they exist in the resultant organoids. Since epithelial regeneration is regulated by tissue mechanics14, it is essential to avoid unintended force generation for self-organizing organoids. In particular, we suggest avoiding contact with the glass bottom of the imaging chamber during live imaging by placing aggregates at the edges of the TEM grids as this allows for free contact with the imaging window of live aggregates (step 5.1.2.). This in vitro–cultured, self-organized 3D model for mucociliary epithelium will serve as a tractable tool to answer the fundamental questions that arise during the regeneration of epithelium and the lineage specification of goblet cells.
The authors have nothing to disclose.
We thank members of Kim lab and Lance Davidson for their comments and support. This work was supported by Young Scientist Fellowship to HYK from Institute for Basic Science (IBS-R0250Y1).
Equipment | |||
Dual-stage Glass Micropipette Puller | Narishige | PC-100 | |
Picoliter microinjector | Warner Instruments | PLI-100A | |
Confocal Laser Microscope | |||
Stereoscope | |||
Tools | |||
Forcep | Dumont | Dumont #5 | |
Hair knife | Reference (Kay, B.K.; Peng, H.B., 1991) | ||
Hair loop | Reference (Kay, B.K.; Peng, H.B., 1991) | ||
hCG injection | |||
human chorionic gonadotropin | Sigma | cg10-10vl | |
MBS solution | |||
10M Sodium hydroxide | Sigma | 72068 | |
Calcium chloride | Sigma | C3881 | |
Calcium nitrate | Sigma | C1396 | |
HEPES | Sigma | H4034 | |
Magnesium sulfate | Sigma | 230391 | |
Phenol-red | Sigma | P0290 | |
Potassium chloride | Sigma | 7447-40-7 | |
Sodium bicarbonate | Sigma | S6014 | |
Sodium chloride | Sigma | S9625 | |
Sodium hydroxide reagent grade, 97%, powder-25g | Sigma | 655104 | |
dejellying solution | |||
L-Cysteine hydrochloride monohydrate | Sigma | C7880 | |
Sodium hydroxide 10M | Sigma | 72068 | |
Ficoll solution | |||
Ficoll | Sigma | F4375 | |
DFA solution | |||
Sodium chloride | Sigma | S9625 | |
0.22mm Filter | Millipore | S2GPT05RE | |
Antibiotic Antimycotic Solution | Sigma | A5955 | |
Bicine | Sigma | B3876 | |
Calcium chloride | Sigma | C3881 | |
Magnesium sulfate | Sigma | 230391 | |
Potassium gluconate | Sigma | G4500 | |
Sodium carbonate | Sigma | 222321 | |
Sodium gluconate | Sigma | G9005 | |
mRNA in vitro transcription | |||
SP6/T7 in vitro transcription kit | Invitrogen | AM1340 | |
mRNA microinjection | |||
Borosilicate glass capillary tubes | Harvard Apparatus | GC100-10 | |
Eppendorf microloader pipette tips | ThermoFisher | A25547 | |
Mineral oil | Sigma | M5904 | |
PCR tube coating | |||
BSA | Thermofisher | 26140079 | |
PCR tubes | SSI | SSI-3245-00 | |
Imaging | |||
Custom-milled acrylic chamber | |||
Coverglass 24mmX50mm | Duran | B01_001650 | |
SPI Hexagonal TEM Grids, Gilded Nickel (50mesh) | SPI | 275HGN-XA | |
SPI Hexagonal TEM Grids, Gilded Nickel (75mesh) | SPI | 2775GN-XA | |
Silicone grease | Shinetsu | HIVAC-G | |
Fixation | |||
20ml screw top-cap vial | Wheaton | WH.986580 | |
2ml screw top-cap vial | |||
Benzyl alcohol | Sigma | 305197 | |
Benzyl benzoate | Sigma | B6630 | |
Dimethyl sulfoxide (DMSO) | Sgima | D4540 | |
Glutaraldehyde 10% EM GRADE | Electron Microscopy Sciences | 16120 | |
Goat serum | Jackson | 005-000-121 | |
Methanol | Sigma | 322415 | |
Paraforlamdehyde | Sigma | P6148 | |
Phosphate-buffered saline (PBS) | LPS Solution | CBP007B | |
Triton X-100 | Sigma | T8787 | |
Primary antibody (1:200) | |||
acetylated tubulin | Sigma | clone 6-11B-1 | |
Itln1 | Proteintech | 11770-1-AP | |
Keratin | Developmental Studies Hybridoma Bank | 1h5 | |
ZO1 | Invitrogen | 402200 | |
Vectors | |||
pCS2-mem-GFP | Gift from Dr. Lance Davidson | ||
pCS2-ZO1-RFP | Gift from Dr. Lance Davidson |