In vitro induction of patterned branchial arch-like aggregate from human pluripotent stem cells

Seto, Yusuke; Ogihara, Ryoma; Takizawa, Kaori; Eiraku, Mototsugu

doi:10.1038/s41467-024-45285-0

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Published: 14 February 2024

In vitro induction of patterned branchial arch-like aggregate from human pluripotent stem cells

Nature Communications volume 15, Article number: 1351 (2024) Cite this article

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Abstract

Early patterning of neural crest cells (NCCs) in the craniofacial primordium is important for subsequent development of proper craniofacial structures. However, because of the complexity of the environment of developing tissues, surveying the early specification and patterning of NCCs is difficult. In this study, we develop a simplified in vitro 3D model using human pluripotent stem cells to analyze the early stages of facial development. In this model, cranial NCC-like cells spontaneously differentiate from neural plate border-like cells into maxillary arch-like mesenchyme after a long-term culture. Upon the addition of EDN1 and BMP4, these aggregates are converted into a mandibular arch-like state. Furthermore, temporary treatment with EDN1 and BMP4 induces the formation of spatially separated domains expressing mandibular and maxillary arch markers within a single aggregate. These results suggest that this in vitro model is useful for determining the mechanisms underlying cell fate specification and patterning during early facial development.

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Introduction

Facial tissues are essential for animal survival; hence, the cellular mechanisms involved in face formation during development are important. The ectoderm-derived cell population called neural crest cells (NCCs) contributes to most of its structures¹. NCCs are transient cell populations that arise from the neural plate border (NPB). They migrate from their dorsal origin to the ventral craniofacial primordium and contribute to the cartilage, bones, and connective tissues.

The facial primordium is divided into three parts, the frontonasal prominence, the maxillary arch (MX), and the mandibular arch (MN), which give rise to distinct parts of the face². As studied extensively in the MN, NCCs are further patterned into several domains characterized by the expression of specific genes in the primordium³. Stepwise patterning events in the early phase are important for proper facial development. However, the underlying cellular processes are not yet fully understood. Environmental signals in which NCCs migrate are thought to be strong determinants of their fate; however, a recent study revealed that the movements of postmigratory NCCs within the mandibular arch are involved in cell fate specification through sequential fate bifurcations³. This study suggests that early-stage patterning of NCCs is a more complicated process than previously thought. Due to technical limitations and the complexity of the environment in developing tissues, there are some difficulties in studying such early processes in vivo. Therefore, we aimed to establish an in vitro model that recapitulates these early processes during facial development.

Human pluripotent stem cells (hPSCs), such as embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSC), have been used for NCC induction in vitro. In the directed differentiation of NCCs from hPSCs^4,5,6,7, balanced WNT/bone morphogenic protein (BMP) signals are important for their induction, similar to NCC induction in model animals⁸. These methods require extrinsic manipulation of signals through the addition of small-molecule regulators. In this study, we established a method to induce a large number of NCC-like cells in a floating culture of hPSC-derived aggregates without additives. NCC induction depends on aggregate-intrinsic WNT/BMP signals. These NCC-like cells were generated from an NPB-like state, and they expressed epithelial-mesenchymal transition (EMT)-related genes during differentiation, indicating that they recapitulated the natural developmental processes. We tested the potential of these aggregates as an in vitro model of early facial development using a long-term culture. Long-term culture-induced MX markers, such as POU3F3 in aggregates⁹. Furthermore, the addition of endothelin 1 (EDN1) and BMP4, secreted from the epithelium surrounding the MN in vivo^10,11, could induce MN markers, such as HAND2, and repress MX markers¹². Temporary treatment with EDN1/BMP4 induced both POU3F3⁺ and HAND2⁺ regions separately within single aggregates, suggesting intrinsic patterning activity in the aggregates. Prolonged culture of the aggregates also permitted the differentiation of cells into osteogenic/chondrogenic lineages. These results indicate its usefulness as an in vitro model for early patterning and cell fate specification in the facial primordium.

Results

Spontaneous induction of NCC-like cells in hPSC aggregates

To induce NCCs from hPSC, we cultured hPSC aggregates in a defined medium (NCC_ind medium) (Fig. 1a). Day-5 aggregates were enriched in SOX10⁺ NCC-like cells. However, they contained a small number of PAX6⁺ neuroepithelial cell (NEC) and CDH1⁺ non-neural epithelium (NNE) cells (Fig. 1b). Knockout serum replacement (KSR), which is often used in neural tissue induction, reduced the induction of NCC-like cells (Supplementary Fig. 1)^13,14. Therefore, we added B-27 supplement instead of KSR to improve cell viability and we analyzed the gene expression during differentiation (Fig. 1c). The hPSC marker NANOG was downregulated on the first day. One of the earliest NCC markers, PAX7, was also upregulated. Subsequently, the early NCC marker FOXD3 and the late marker SOX10 were upregulated. We confirmed the expression of these genes using immunohistochemistry (IHC) (Fig. 1d). Under our experimental conditions, we detected cells co-expressing all these markers in the peripheral region of the aggregates from day 3. The expression of FOXD3 and SOX10 mostly overlapped at every stage tested, although the expression of FOXD3 seemed to be weaker on day 5 than on day 4. In model animals, such as chicks and zebrafish, FOXD3 and SOX10 are expressed in distinct but partially overlapping patterns, and FOXD3 seems to be downregulated as development proceeds^15,16. Weaker expression of FOXD3 at day 5 may reflect the downregulation observed in vivo. The expression of PAX7 was broader than that of the others, especially on day 4 (Fig. 1d, e). FOXD3^-/SOX10^-/PAX7⁺ cells might be earlier NCC precursor-like cells or other types of cells, such as dorsal NECs¹⁷.

**Fig. 1: Induction of NCC (neural crest cell)-like cells in the aggregates derived from hPSCs (human pluripotent stem cells).**

As day-5 aggregates contained non-NCC-type cells (Fig. 1b), we analyzed the content of aggregates derived from KhES-1 cells by using single-cell RNA-seq (scRNA-seq). A total of 1877 cells were divided into 14 clusters using unsupervised uniform manifold approximation and projection (UMAP) clustering (Fig. 2a, b). Clusters 0, 1, 3, 4, 5, 6, and 7 represented SOX10⁺ NCC-like cells, accounting for 71.9% of the total cells (Fig. 2c, d). The expression of PAX7 and FOXD3 was higher in cluster 5 than in the NCC-like clusters, indicating that cluster 5 represented nascent NCC-like cells (Fig. 2e, g). The presence of PAX7^high/FOXD3^high cells was confirmed using IHC (Fig. 1d). Cluster 8, adjacent to cluster 5, represented SP5⁺/PAX7 ⁺/FOXD3⁺ NPB-like cells (Fig. 2f, g)¹⁸. Cluster 8 was also characterized by extremely low SOX10 expression (Fig. 2b, d, g). Cluster 9 represented more immature cells, characterized by higher POU5F1 expression and the absence of SOX17 expression, a marker gene for endodermal cells (Endo) (Fig. 2h)¹⁹. Cluster 10 contained CDH1⁺/TFAP2A⁺ NNE-like cells, which also contained SIX1⁺ pre-placodal epithelium (PPE)-like cells (Fig. 2i)²⁰. Three types of neural lineage clusters were observed (Fig. 2j). Cluster 2 appeared to represent ISL1⁺/SIX1⁺ NCC-derived neuron-like cells (Fig. 2i, j)²¹ and cluster 11 contained PAX6⁺/EMX2⁺ NEC-like cells (Fig. 2j)²². Cluster 13 comprised EBF2⁺ placode-derived neuron-like cells (Fig. 2j)²³. Cluster 12 represented SOX17⁺ endo-like cells (Fig.2h). We also confirmed the presence of these cell types using IHC (Supplementary Fig. 2).

**Fig. 2: Analysis of the cellular content of the day-5 aggregates.**

Although p75 is not specific to NCCs in human embryos²⁴, scRNA-seq revealed that p75 expression was mostly specific to NCC-like cells in day-5 aggregates (Supplementary Fig. 3a). IHC also showed that p75 expression was specific to SOX10⁺ cells (Supplementary Fig. 3b, c). Almost consistent with scRNA-seq data, 76.5% of the cells in day-5 aggregates derived from KhES-1 expressed p75 at a high level on average (Supplementary Fig. 3d). Although the induction efficiency of SOX10⁺ cells varied among the hPSC lines, the expression of p75 appeared to be specific to SOX10⁺ cells in all cell lines (Supplementary Fig. 3b–d). These results indicate that p75 could be a suitable surface antigen for isolating NCC-like cells in this experimental system. We also tested the multipotency of these NCC-like cells (Supplementary Fig. 4). These cells can differentiate into neuronal cells, glial cells, melanoblasts, and ectomesenchymal derivatives, including smooth muscle cells, osteoblasts, chondroblasts, and adipocytes.

In our method, NCC-like cells were spontaneously induced without additives, such as CHIR-99021, BMPs, and SB-431542 (SB) that are often used in other methods^4,5,6,7, indicating the importance of aggregate-intrinsic signals. We analyzed the role of aggregate-intrinsic WNT/BMP signals, considering their importance in vivo (Fig. 3a)⁸. WNT1/4 were expressed in aggregates and peaked on days 3 and 4. AXIN2, a well-known downstream target of the canonical WNT signal²⁵, also peaked around days 3 and 4. A non-canonical WNT, WNT7B, was upregulated on day 1. BMP2 was also upregulated around day 3, whereas BMP4 was upregulated between days 0 and 1, suggesting that BMP4 may play a role in the early phase of differentiation. To reveal the function of WNT/BMP signals in our method, we treated the aggregates with the WNT inhibitor IWP-2 or the BMP inhibitor dorsomorphin (Fig. 3b). We added these inhibitors from days 1, 2, or 3 to day 5 and analyzed the expression of marker genes for NCC-like cells (SOX10), NEC-like cells (PAX6), NNE-like cells (CDH1), and PPE-like cells (EYA1) (Fig. 3c). WNT inhibition starting on day 1 strongly suppressed SOX10 (Fig. 3c). This effect was weakened when IWP-2 was added on day 2. Treatment from day 3 showed almost no effect, suggesting that WNT signaling before day 3 was important for NCC-like cell induction. However, canonical WNT activity was upregulated from day 3, suggesting the importance of the early expression of non-canonical WNTs, such as WNT7B. BMP inhibition on day 1 also suppressed SOX10 expression, although this suppression was incomplete. This indicates that BMPs, such as BMP4, might act from day 0, and that their activities were sufficient to induce the differentiation of subsets of NCC-like cells. BMP inhibition on day 3 was also ineffective on SOX10 expression. For PAX6, CDH1, and EYA1, both WNT and BMP inhibition from day 3 had almost no effect, indicating that the fate of most precursors was determined by day 3 (Fig. 3c). WNT inhibition from days 1 and 2 slightly upregulated CDH1 and EYA1, indicating an increase in the number of NNE/PPE-like cells. Conversely, BMP inhibition on day 1 significantly reduced the expression of CDH1 and EYA1, indicating a reduction in the number of NNE/PPE-like cells. BMP inhibition from day 1 also resulted in the upregulation of PAX6, indicating an increase in the number of NEC-like cells, which was consistent with a reduction in NNE/PPE marker expression. Interestingly, WNT inhibition from days 1 and 2 also upregulated PAX6, although NNE/PPE markers were also upregulated under these conditions. We analyzed the aggregates treated with inhibitors from day 1 using IHC (Fig. 3d, e). Consistent with the results of real-time polymerase chain reaction (PCR), an increase in the number of PAX6⁺ cells and a decrease in those of SOX10⁺ and CDH1⁺ cells were observed in the dorsomorphin-treated aggregates. In contrast, SOX10⁺ cells disappeared completely from the IWP-2-treated aggregates (Fig. 3d). In IWP-2-treated aggregates, the counts of PAX6⁺ cells were increased, as well as in dorsomorphin-treated aggregates, but these PAX6⁺ cells expressed CDH1, suggesting that they were of the NNE/PPE-like lineage. PAX6 is also expressed in the anterior PPE, such as the lens/olfactory placodes²⁶. We confirmed that there were SIX1⁺/CDH1⁺ cells that did not express PAX6 in non-treated aggregates, while many SIX1⁺/CDH1⁺ cells co-expressed PAX6 in IWP-2-treated aggregates, suggesting that they became anterior PPE-like cells (Fig. 3e). Anteriorization by WNT inhibition is consistent with the role of WNT signals in the posteriorization of embryonic ectodermal tissue during early development²⁷. For dorsomorphin-treated aggregates, there were hardly any CDH1⁺ NNE/PPE-like cells. These results indicate that aggregate-intrinsic WNT and BMP signals regulate the positional identity of cells to induce NCC-like cells (Fig. 3f).

**Fig. 3: Importance of endogenous WNT/BMP (bone morphogenetic protein) signals during NCC (neural crest cell)-like cell induction.**

SB, which antagonizes activin/TGFß signal, was also used in some NCC induction methods^4,6. We tested the effect of SB on our NCC-like cell induction method by adding SB from day 0 onward. In SB-treated aggregates, the number of NCC-like cells expressing FOXD3 and SOX10 increased, while SOX17⁺ endo-like cells disappeared on day 4 (Supplementary Fig. 5). This result is consistent with the finding that SB inhibits the endodermal induction of hPSCs while facilitating neuroectodermal differentiation²⁸.

Recapitulation of in vivo differentiation process during NCC-like cell induction

We appended the scRNA-seq data of day-2, day-3, and day-4 aggregates to the data of day-5 aggregates for further analysis of the developmental process of the aggregate (total 8466 cells, 23 clusters) (Fig. 4a). The cells in the cluster 15 were POU5F1⁺/NANOG⁺/EOMES^-, indicating that they exhibited a PSC-like state (Fig. 4b)²⁹. The early NPB marker TFAP2A, which can induce the expression of NCC marker genes when overexpressed in hPSCs³⁰, was upregulated in cluster 7 adjacent to cluster 15, and its expression continued throughout differentiation into NCC-like and NNE/PPE-like cells (Fig. 4c). The NPB markers, SP5 and IRX1, were upregulated in groups of clusters adjacent to cluster 7 (clusters 3, 6, 8, 9, 13, 14, 16, and 19) (Fig. 4d)^18,31. Another group of clusters (clusters 1, 2, 4, 5, 11, 17, and 20) contained SOX10⁺ NCC-like cells (Supplementary Fig. 6). In this group, some marker genes showed differential expression patterns, suggesting variations in cell state (Supplementary Fig. 6). Cluster 0 was the transition point between the NPB-like and NCC-like clusters expressing FOXD3 and SOX9 at high levels (Fig. 4e and Supplementary Fig. 6). Cluster 10 contained SIX1⁺ NCC-derived neuron-like cells (Fig. 4f). Clusters 12/18 contained KRT18 ^high NNE/PPE-like cells, and cluster 22 contained PAX6⁺ NEC-like cells (Fig. 4f). Cluster 21 contained SOX17 ⁺ endo-like cells (Fig. 4g). RNA velocity analysis suggested that most of the hPSCs differentiated into NPB-like states. However, only a small number of cells differentiated into SOX17⁺ endo-like cells (Fig. 4h). NPB-like cells seemed to differentiate into NCC-like-, NEC-like-, and NNE/PPE-like cells, suggesting that NPB-like cells are multipotent, as observed in model animals (Fig. 4h)^32,33,34.

**Fig. 4: Characterization of the developmental process of the aggregates.**

NPB is located between SOX2⁺ NE and TFAP2A⁺ NNE, and both genes are expressed in model animals³². In our scRNA-seq dataset, we observed the co-expression of SOX2 and TFAP2A NPB-like clusters at all stages analyzed (Supplementary Fig. 7a). We also confirmed co-expression in the aggregates using IHC (Supplementary Fig. 7b). Co-expression of SOX2 and TFAP2A clearly appeared on day 2. However, TFAP2A was already expressed in some SOX2⁺ cells weakly at day 1. From day 4, SOX2 expression decreased in most TFAP2A⁺ cells, including NPB-like, NCC-like, and NNE/PPE-like cells. This downregulation of SOX2 has been observed in the NCCs of chick embryos³². Some SOX2⁺/TFAP2A⁺ cells also co-expressed PAX7 from day 3, similar to the NPB cells in chick embryos (Supplementary Fig. 7b).

We analyzed the NPB clusters in more detail to identify the variety in the cell state. NPB clusters (clusters 3, 6, 7, 8, 9, 12, 13, 14, 16, and 19) were extracted according to the expression of SP5 and TFAP2A and re-clustered to identify differentially expressed genes (Supplementary Fig. 8a). SP5 and IRX1 were broadly expressed. The cranial NCC (cNCC) markers, LHX5, DMBX1, and OTX2, were also expressed in a large number of cells, although the expression patterns of DMBX1 and OTX2 were sparse and restricted, compared with LHX5, respectively. Lhx5 and Dmbx1 are regulators of the cNCC-specific gene regulatory network in chick embryo³⁵. Otx2 is crucial for head development³⁶. These results suggest that most NPB-like cells possess a cNCC-like identity.

The NPB-like cell clusters could be roughly classified into two groups (Supplementary Fig. 8c, d). One group expressed genes related to the anterior patterning of the embryonic ectoderm, such as CYP26A1 and HESX1^37,38. This group mainly comprised cells of early differentiation days (day 2: 85.0%, day 3: 14.6%, day 3: 0.4%, and day 4: 0%). In the other group, NPB-specified PAX3 and MSX1 were broadly expressed instead of CYP26A1 and HESX1. This group comprised cells from late differentiation days (day 2: 13.8%, day 3: 26.9%, day 3: 29.6%, and day 4: 29.7%). This result suggests that early anterior patterning of the ectoderm occurred in the aggregates, followed by further differentiation of NPB-like cells.

We also searched for genes with distinct expression patterns within NPB-like cell clusters, focusing on transcription factors (Supplementary Fig. 9). For example, FOXD3, ZFHX4, TFAP2B, DACH1, and EBF1 are expressed in cells located on the left side of the UMAP. These genes are also expressed in NCCs in vivo^39,40,41,42. PAX6, EN1, PAX2, PAX7, OTX2, GBX2, and EMX2, which are essential factors for the patterning of NECs around the diencephalon and mid/hindbrain, also exhibited distinct patterns^22,43, suggesting those factors may define the axial identity of NPB-like cells in the aggregates. DLX5, GRHL2, GRHL3, MAFB, and HAND1 also showed characteristic expression patterns in NPB-like cells, and their expression extended to NNE/PPE-like cell clusters. These genes are involved in the NNE/PPE lineage development in vivo^44,45,46,47. The tumor suppressor ELF3 is also expressed in similar patterns, although its role in NPB-like cells is unknown⁴⁸.

Furthermore, a trajectory analysis of NPB-like cells was performed to identify gene expression patterns specific to the trajectories bound for each ectodermal lineage (Supplementary Fig. 10a). Some genes exhibited differential expression patterns among the different lineages (Supplementary Fig. 10b). These genes included those known to be important for the development of each lineage, as earlier mentioned. There were also several lineage-specific genes whose roles in the differentiation of NPB-like cells were unknown, such as EBF1 and SEZ6 for the NCC-like lineage and ELF3, KLF6 for the NNE/PPE-like lineage. For example, EBF1 is known for its expression in migratory cNCC, but its function in NPB is unknown⁴¹, similar to that of ELF3.

To infer the gene expression dynamics during the differentiation of NCC-like cells, pseudotime analysis was performed on clusters, excluding NNE/PPE-like, NEC-like, and endo-like cell clusters (Fig. 4i). Markers for PSC, NPB, and NCC were upregulated along with pseudotime, indicating the recapitulation of the in vivo NCC induction processes to some extent (Fig. 4j). During in vivo development, EMT is associated with the induction of NCCs as they leave their origin. EMT is characterized by several features, such as the expression of EMT-inducing transcription factors, changes in the expression of cadherin genes, and degradation of the basement membrane⁴⁹. Gene ontology analysis of cluster markers revealed that in marker genes for cluster 0 of the merged scRNA-seq data, which was a junction between NPB and NCC-like cells, were enriched with genes related to EMT (i.e., GO:0001837 “epithelial to mesenchymal transition”) (Supplementary Fig. 11). Pseudotime analysis revealed temporal changes in the expression of several genes related to EMT, including transcription factors and cadherins (Fig. 5a). Consistent with their regulatory relationships, the expression of PAX3/7 and MSX1 was higher than that of FOXD3, ETS1, and SNAI2 ⁵⁰. Furthermore, the downregulation of CDH2 coincided with the expression of SNAI2 and LMO4⁵⁰. Changes in cadherin expression in the aggregates were analyzed by IHC. Epithelial cadherin CDH1 was expressed in the aggregates almost entirely on day 2 and then gradually restricted as differentiation proceeded (Fig. 5b). Most CDH1^- regions were occupied with FOXD3⁺ cells. SOX10⁺ cells in day-4 aggregates expressed the EMT-related cadherin CDH6 with a complementary pattern to CDH1, indicating the switching of cadherins during NCC-like cell differentiation (Fig. 5c). These results suggest that the in vitro induction of NCC-like cells may recapitulate the in vivo developmental processes to some extent (Fig. 5d).

**Fig. 5: Recapitulation of gene expression changes associated with EMT (epithelial-mesenchymal transition).**

Induction of branchial arch mesenchyme-like aggregates with specific regional identity

We tested the potential of these aggregates as in vitro models of early craniofacial development. As earlier mentioned, the expression of DMBX1, LHX5, OTX2 (Supplementary Fig. 8b), and ETS1 (Fig. 5a) suggested that the induced NCC-like cells possessed a cranial identity (Fig. 5a, Supplementary Fig. 8b)^35,36. cNCCs migrating into future craniofacial regions are HOX-negative in vivo⁵¹. We were unable to detect HOX-positive NCC-like cells in the aggregates, indicating that the cells in the aggregates possessed the ability to differentiate into the craniofacial mesenchyme (Supplementary Fig. 12).

We performed a long-term culture of the aggregates to induce craniofacial mesenchyme-like cells. The addition of basic fibroblast growth factor (bFGF) from day 4 enabled long-term culture, preventing the collapse of the aggregates during the culture period (Fig. 6a). bFGF-treated aggregates expressed the craniofacial mesenchyme markers PRRX1 and TWIST1 at higher levels than untreated aggregates (Fig. 6b)⁵². IHC of day-28 aggregates revealed that many cells expressed these genes, indicating the induction of craniofacial mesenchymal-like cells (Fig. 6d, e). The branchial arch markers, DLX2 and BARX1, were also expressed in day-28 aggregates (Fig. 6b, e, f)⁵³. These aggregates also expressed LHX6, a specific marker expressed on the oral side of the first branchial arch mesenchyme in a spatially restricted manner, as observed in vivo (Fig. 6b, e)⁵⁴. The first branchial arch is composed of the MX and MN, which give rise to the upper and lower jaws, respectively. bFGF-treated aggregates expressed POU3F3, which was strongly expressed in early MX⁹, but did not express the MN marker DLX5 (Fig. 6b and Supplementary Fig. 13), indicating that bFGF-treated aggregates acquired an MX-like identity (Fig. 6c).

**Fig. 6: Differentiation of the aggregates into craniofacial mesenchyme-like cells.**

The choice of fate between MX and MN is regulated by the peptide hormone EDN1^10,55. EDN1 is secreted from the epithelium surrounding the MN and is essential for the induction of MN specifiers, such as DLX5/6 (Fig. 7a). Loss of the EDN1 signal results in the transformation of the lower jaw into an upper jaw-like structure¹⁰, whereas overactivation transforms the MX into an MN-like identity⁵⁵. However, the NCC-like cells in the aggregates expressed the EDN1 receptor EDNRA (Fig. 7b), and the aggregates comprised only a small number of NNE-like cells, a possible source of EDN1. We hypothesized that low levels of EDN1 in the aggregates would result in the acquisition of an MX-like identity during long-term culture. Therefore, we cultured the aggregates with or without EDN1 and BMP4 from day 4 in the presence of bFGF (Fig. 7c). BMP4 is also secreted from the craniofacial epithelium and is important for the patterning of MN¹¹. When EDN1 and BMP4 were added, POU3F3 was downregulated, whereas DLX5 was upregulated (Fig. 7d). DLX5 expression was higher in the aggregates treated with both EDN1 and BMP4 than in those treated with either EDN1 or BMP4 alone. However, the cooperative effect of EDN1 and BMP4 on DLX5 expression has not been reported previously.

**Fig. 7: Mandibular arch (MN)-like differentiation of the aggregates.**

The MN is divided into several subdomains (Fig. 7e). We analyzed the expression of domain-specific genes to characterize the MN-like cells induced by EDN1/BMP4 (Fig. 7f). EDN1/BMP4-treated aggregates expressed the distal region markers HAND2 and MSX2. However, MSX2 expression was upregulated when BMP4 was added alone. EDN1/BMP4-treated aggregates also expressed the intermediate region markers DLX3 and NKX3.2⁵⁶. Regarding the oral-aboral axis, the aboral marker GSC was upregulated in the EDN1/BMP4-treated aggregates, whereas the oral side marker LHX6 was downregulated. Furthermore, in the EDN1/BMP4-treated aggregates, WNT5A, which regulates the proper morphogenesis of MN, was also upregulated⁵⁷. We confirmed the expression of these markers in EDN1/BMP4-treated aggregates using IHC (Fig. 7h, i). These results suggested that the aggregates were similar to the in vivo branchial arch mesenchyme in their responsiveness to signaling factors, and that EDN1 and BMP4 cooperatively induced the identity of the aboral side of the distal, intermediate regions of the MN (Fig. 7g).

Formation of the patterned branchial arch-like aggregates through temporary treatment

The MX and MN can be recognized as the dorsal and ventral sides of the same tissue, as the most dorsal side is POU3F3⁺ MX and the ventral side is HAND2⁺ region of the MN (Fig. 8a). At day14, in bFGF-treated aggregates that differentiated into an MX-like state, most NCC cells still expressed SOX10 (Supplementary Fig. 14). In contrast, in EDN1/BMP4-treated aggregates that differentiated into an MN-like state, most mesenchymal cells expressed DLX5 instead of SOX10, indicating fate commitment into the MN-like lineage. However, in these aggregates, a subset of cells still expressed SOX10, suggesting that they were not committed to the MN-like lineage and could differentiate into the MX-like state if EDN1 and BMP4 were removed from the medium. We tested whether single aggregates contained both POU3F3⁺ (dorsal MX-like) and HAND2⁺ (ventral MN-like) cells by removing EDN1 and BMP4 from the medium on day14 (Fig. 8b). The temporarily treated aggregates (condition 2 in Fig. 8b) expressed both POU3F3 and HAND2 at higher levels than the EDN1/BMP4-treated (condition 3 in Fig. 8b) and non-treated aggregates (condition 1 in Fig. 8b) on day 21(Fig. 8c). Furthermore, POU3F3⁺ and HAND2⁺ cells formed spatially separated domains in temporarily treated aggregates, indicating mutually exclusive fate choice and self-patterning properties of cells (Fig. 8d).

**Fig. 8: Formation of the 1st BA (first branchial arch)-like aggregates containing both proximal and distal-type cells.**

We analyzed previously reported scRNA-seq data from the mouse branchial arch on embryonic day 9.5 (E9.5) as a reference⁵⁸. Hox-negative cells were extracted from NCC clusters and re-clustered (Supplementary Fig. 15a). There were clusters positive for the MN markers Dlx5 and Hand2 (clusters 1, 2, and 7) and clusters positive for the MX arch marker Pou3f3 (clusters 0, 3, 5, and 6). Several presumptive markers for these clusters were identified (Supplementary Fig. 15b). On day 21, EDN1/BMP4-untreated aggregates expressed the presumptive MX arch markers EBF1 and POU3F1 at higher levels, and EDN1/BMP4-treated aggregates expressed the presumptive MN marker DLK1 at higher levels (Supplementary Fig. 15c). Temporarily treated aggregates expressed high levels of both markers. The scRNA-seq of E9.5 mouse embryo showed that Pou3f3 and Hand2⁺ were expressed in a mutually exclusive manner (Supplementary Fig. 15d). IHC also showed the separation of Pou3f3⁺ and Hand2⁺ cells in the first branchial arch at E9.5, as in case of temporarily treated aggregates (Supplementary Fig. 15e).

As the branchial arch mesenchyme differentiates into craniofacial bones and cartilage in vivo¹, we analyzed the osteogenic/chondrogenic potential of the aggregates by extending the culture period (Fig. 9a). On day 35, temporarily treated aggregates expressed RUNX2, a marker for osteogenic/chondrogenic lineage cells (Fig. 9b)⁵⁹. Some RUNX2⁺ cells express SP7, an osteogenic cell marker, indicating their differentiation into this lineage⁵⁹. Furthermore, in the aggregates, there were cell condensates positive for SOX9 and NKX3.2, which were expressed in the Meckel’s cartilage of mouse embryos (Fig. 9c)^60,61. These results suggest spontaneous differentiation of NCC-like cells into osteogenic/chondrogenic lineages in culture.

**Fig. 9: Osteogenic/Chondrogenic differentiation in the aggregates after long-term culture.**

Discussion

In this study, we established a method for inducing aggregates enriched with NCC-like cells to differentiate into a brachial arch-like state. In this method, NCC-like cells can be induced through a simple procedure within 5 days with efficiencies of 45.4–76.5% depending on the cell lines (Supplementary Fig. 3c). Aggregate-intrinsic WNT/BMP signaling can induce NCC-like cells in an NPB-like state. NPB-like cells also differentiate into other ectodermal lineages, including NEC-like-and NNE/PPE-like cells. This feature may be suitable for investigating the mechanisms underlying the fate choice in multipotent NPB-like cells in a natural context.

Although the expression of several WNTs and BMPs was upregulated from differentiation day 3 in the aggregates, the inhibition of these signals from day 3 did not affect the induction of NCC-like cells. We could not determine the cause of this ineffectiveness of Wnt/BMP inhibition from day 3; however, the autoregulation of NCC specifiers could possibly facilitate the differentiation of NCC-like cells in the absence of Wnt/BMP signals after day 3. For example, Pax3 specifies NCC identity in Xenopus animal caps and maintains its expression through autoregulatory feedback⁶². Based on our scRNA-seq data, only 27.7% of NPB-like cells in day-2 aggregates expressed PAX3, and 86.5% in the day-3 aggregates (Supplementary Fig. 16a). Although Pax3 required Zic1 to activate the complete NCC developmental program in Xenopus⁶², most NPB-like cells in the aggregates expressed the ZIC2/3/5 family genes (Supplementary Fig. 16b). Zic2/3/5 are also important for the development of NCCs in model animals^63,64,65. Considering these factors, it is possible that NCC specifiers, such as PAX3 and ZIC2/3/5, which were already expressed by day 3, could promote the differentiation of NPC-like cells into NCC-like cells independently of WNT/BMP signals. As the WNT signal regulates multipotency of NCCs through the Lin28/let-7 axis in chick⁶⁶, it is possible that NCC-like cells in the aggregates treated with IWP-2 from day 3 have deficits in terms of differentiation potential, although we have not checked that possibility yet. As for BMP signals, reporter assays using BMP-responsive elements have revealed that the activity of the BMP signal was relatively low during the specification of NCCs and was upregulated during EMT in chick⁶⁷. This was consistent with the ineffectiveness of BMP inhibition from day 3 on NCC-like cell induction in the aggregates.

We also analyzed the effect of activin/TGFß inhibition by SB in our protocol. NCC induction methods based on dual-SMAD inhibition use SB as an important component, but SB showed adverse effects on NCC induction, as reported by Leung and colleagues⁵. In our protocol, SB had a positive impact on NCC induction, as indicated by the enhanced generation of FOXD3⁺ cells and the inhibition of SOX17⁺ endoderm induction on day 4. It is important to elucidate the cause of the difference in the response to SB to understand the mechanisms behind the induction of NCC from hPSCs.

scRNA-seq analysis suggested a possible lineage bifurcation from the NPB-like state to the NCC-like, NEC-like, and NNE/PPE-like states in the aggregate, which is consistent with the fact that NPB cells can contribute to all ectodermal lineages in model animals^32,33,34. We identified genes that were upregulated during the differentiation into specific lineages. However, the functions of some of these genes in NPB-like cells have not yet been elucidated. Further analysis of these uncharacterized genes would provide a more precise understanding of the lineage specifications in multipotent NPB-like cells.

Long-term culture of the aggregates demonstrated their potential to differentiate into an MX-like state in the presence of bFGF. The aggregates tended to collapse during long-term culture without bFGF, consistent with the previous observation that mesenchymal cells in the first branchial arch require the FGF signal to survive⁵⁴.

MX is transformed into an MN-like state upon receiving an EDN1 signal in vivo⁵⁵. The aggregates express marker genes for MN, such as DLX5 and HAND2, when stimulated with EDN1 and BMP4. The cooperative effect of EDN1 and BMP4 on DLX5 expression has not been observed in previous in vivo studies, indicating an advantage of this in vitro model over in vivo models.

Temporary treatment with EDN1 and BMP4 induced the formation of both HAND2⁺ and POU3F3⁺ cells within single aggregates. Although the mechanism underlying the co-emergence of HAND2⁺ cells and POU3F3⁺ cells has not yet been identified, it is possible that fate-uncommitted NCC-like cells in day-14 EDN1/BMP4-treated aggregates differentiated into an MX-like state after removing EDN1 and BMP4 from the medium (Supplementary Fig. 14). Although EDN1 and BMP4 were both-applied to the medium, HAND2⁺ and POU3F3⁺ cells did not intermingle but segregated spatially, forming distinct domains inside the aggregates. The mechanisms underlying the segregation of the two cell types have not yet been elucidated. The gathering of the same type of cells through a chemoattractant and dominant adhesion between these cells might be considered possible mechanisms. For example, the NCC-like cells expressed WNT5A after stimulation with EDN1/BMP4 (Fig. 7f). WNT5A acts as a chemoattractant⁶⁸. If MN-type cells respond selectively to WNT5A, the chemotactic properties of WNT5A may cause MN-type cells to form clusters in aggregates. In the EDN1/BMP4-treated aggregates, DLX5⁺ cells expressed CDH11 strongly on day 7 (Supplementary Fig. 17). CDH11 expression appears to be upregulated in the MN, MX, and limbs at the onset of their morphogenesis⁶⁹. CDH11 exhibits homophilic binding when expressed in L cells, and CDH11⁺ cells are sorted from the negative population in aggregates derived from the dissociated limb bud mesenchyme⁶⁹. Other factors beyond CDH11 may be at play; moreover, such a homophilic cell adhesion-dependent mechanism might also be involved in the formation of HAND2⁺ and POU3F3⁺ regions in the aggregates.

Finally, differentiation into the osteogenic/chondrogenic lineages was observed in the aggregates, expanding the applicability of this model. Although the important genes and signals for commitment to these lineages have been identified⁷⁰, the actual processes during cell fate decisions are not yet properly understood. Using appropriate reporter cell lines, the dynamics of commitment to the osteogenic or chondrogenic lineage can be visualized. Such experiments could also provide insights into the cellular mechanisms underlying craniofacial development.

Methods

Maintenance and differentiation of hPSCs

StemFit AK02N (Reprocell) was used as a maintenance medium for hPSCs in feeder-free cultures. Three PSC lines were used in the experiments. Two ESC lines (KhES-1 and KthES11) and one iPSC line (253G1). For passage, the cells were dissociated into single cells by treatment with 0.5 × TrypLE Select (Gibco) diluted with 0.5 mM EDTA/phosphate-buffered saline (PBS) (−Ca²⁺, −Mg²⁺). Dissociated cells were counted and 1.6 × 10⁴ cells were plated into the wells of 6-well culture plates coated with iMatrix-511 (Nippi). Y-27632 was added (10 μM) on the first day of plating. For differentiation, the dissociated cells were suspended in NCC_ind medium containing Y-27632 (1.0 × 10⁴ cells per 100 µl per well) and re-aggregated in 96-well V-bottom low-cell-adhesion plates (Sumitomo Bakelite). This day was defined as day 0. Briefly, 50 µl NCC_ind medium not containing Y-27632 was added to each well on day 1 and half of the medium was replaced with fresh medium on day 3.

For long-term culture, 50 µl of NCC_ind medium containing bFGF was added on day 4. Subsequently, half of the medium was replaced with the bFGF-containing NCC_ind medium every other day. In some experiments, EDN1 and BMP4 were added from day 4 onward. For temporary treatment with EDN1 and BMP4, the aggregates were washed with fresh NCC_ind medium and cultured in bFGF-containing NCC_ind medium.

The NCC_ind medium was DMEM/F-12 with GlutaMAX (Gibco) supplemented with 1 × B-27 minus vitamin A (Gibco) and 5 mg/ml bovine serum albumin (BSA; Fujifilm). KSR (Invitrogen) was added as required.

For neuronal, glial, and melanocytic differentiation, cells dissociated from Day-5 aggregates were plated on iMatrix-coated glass-bottom dishes (1.0 × 10⁴ cells/cm²) and cultured in NCC_ind medium. Aggregates were dissociated using TrypLE Express (Gibco). For differentiation into smooth muscle cells, the cells were plated on glass-bottomed dishes coated with Geltrex (A1413202; Gibco). For differentiation into osteogenic, chondrogenic, and adipogenic cells, dissociated cells were cultured on Geltrex-coated dishes (1.3 × 10⁴ cells/cm²) in NCC_ind medium containing bFGF for 6 days. The cells were dissociated using Accumax (Nacalai Tesque) and used in subsequent cultures. For osteogenic differentiation, the cells were plated on Geltrex-coated dishes (1.0 × 10⁴ cells/cm²) and cultured with the contents of the StemPro Osteogenesis Differentiation Kit (Gibco). For chondrogenic differentiation, the dissociated cells were aggregated in 96-well V-bottom low-cell-adhesion plates (3 × 10⁵ cells/well) and cultured using the contents of the StemPro Chondrogenesis Differentiation Kit (Gibco). For adipogenic differentiation, the cells were plated on Geltrex-coated dishes (1.0 × 10⁴ cells/cm²) and cultured with the contents of the StemPro Adipogenesis Differentiation Kit (Gibco).

Bright-field images were acquired using a BZ-X710 microscope with the BZ-X Viewer (Version 1.4.0.1, KEYENCE).

The concentrations of reagents in the culture medium during the experiment were as follows: Y-27632 (10 µM; Tocris), dorsomorphin (2 µM; R&D Systems), IWP-2 (5 µM; Selleck Biotech), SB-431542 (10 µM; Selleck Biotech), bFGF (20 ng/ml; R&D Systems), EDN1 (25 ng/ml; Peptide Institute, Inc.), and BMP4 (5 ng/ml; R&D Systems).

The experimental protocols were approved by the Research Ethical Committee of Kyoto University.

Immunohistochemistry and other staining

For sectioning of the aggregates, the aggregates were fixed with 4% paraformaldehyde/PBS (-) at 4 °C for 15 min, washed with PBS, and immersed in 15% sucrose/PBS (-) at 4 °C overnight. The fixed aggregates were frozen in O.C.T. compound (Sakura Finetek) and sectioned using a cryostat (10 µm sections). The sections were permeabilized with 0.3% Triton X-100/PBS (-) for 20 min at room temperature and then blocked with Blocking-One (Nacalai Tesque) for 30 min at room temperature. The primary antibodies were diluted in Blocking-One and incubated overnight at 4 °C with the sections. After washing, the sections were incubated with diluted secondary antibodies (Donkey-raised antibodies conjugated with Alexa Fluor 488 or Cyanine Cy3 or Cyanine Cy5, Jackson ImmunoResearch Labs) and 4′,6-diamidino-2-phenylindole (DAPI; Nacalai Tesque) for 2 h at room temperature. A TCS SP8 confocal microscope and the Leica Application Suite X software (Version 3.5.5.19976, Leica) was used to capture the images. ImageJ (1.53k) was used to merge single-channel images. The dissociated cells cultured in glass-bottomed dishes were stained in a similar manner. Cryoprotection and sectioning steps were omitted. For preparing mouse embryonic tissue, fixation period was prolonged to 2 h, and cryoprotection was performed with 30% sucrose/PBS (-). The antibodies used in this study are listed in Tables S1 and S2.

For Alizarin Red staining and Oil Red O staining, the cells were fixed in 4% paraformaldehyde/PBS (-) for 15 min at room temperature. For Alcian Blue staining, the aggregates were fixed in 4% paraformaldehyde/PBS (-) at 4 °C for 15 min and cryoprotection and sectioning were performed in the same manners with IHC samples.

All animal procedures comply with all relevant ethical regulations and guideline for animal studies approved by the Research Ethical Committee of Kyoto University. Pregnant mice (Slc:ICR) were obtained from SHIMIZU Laboratory Supplies Co., Ltd. The mice were maintained on a 12-h light/dark cycle at room temperature (24 ± 2 °C) with constant humidity (55 ± 10%).

Flow cytometry analysis

The aggregates were pooled and dissociated using TrypLE Express (Gibco) at 37 °C for 8 min. The reaction was stopped by adding PBS, and the cells were filtered through a cell strainer. Dissociated cells were stained with an allophycocyanin-conjugated anti-p75 antibody or isotype control (1:50; Miltenyi Biotec) according to the manufacturer’s instructions. After washing, the cells were counted using a FACSAria IIIu flow cytometer (Becton Dickinson). The data were analyzed using the FACSDiva software (Version 6.1.3, Becton Dickinson). The FlowJo software (Version 10.8.1, Becton Dickinson) was used to make histograms. The antibodies used in this study are listed in Table S3.

RNA isolation and real-time PCR

Total RNA was purified from 24 cell aggregates per condition using the RNeasy Micro Kit (QIAGEN), and cDNA was synthesized using SuperScript II (Invitrogen). Real-time PCR was performed using Power SYBR Green PCR Master Mix and QuantStudio 6 and 7 Flex Real-Time PCR Systems (Version 1.3, Applied Biosystems). All data were analyzed using the standard curve method and GAPDH expression was used for normalization. The primer sequences are summarized in Table S3.

scRNA-seq and data analysis

Aggregates derived from KhES-1 were used for scRNA-seq experiments. For sample preparation, 32 aggregates were pooled and dissociated using TrypLE Express for 8 min at 37 °C. Gentle pipetting with a low-binding tip was performed 4 and 8 min after incubation. An equal amount of 0.04% BSA/PBS (-) was added to the dissociated cells, and the cell suspension was filtered through a cell strainer (Falcon). The cells were counted and pelleted through centrifugation at 430 × g for 5 min. The cells were resuspended in 0.04% BSA/PBS (-) (adjusted to 1000 cells/µl) and placed on ice until use. About 5000 cells per sample were used in library construction using v3 Chromium Single Cell 3′ Reagent Kits (10X Genomics). Gel bead-in-emulsion generation, cDNA synthesis, and library preparation were performed according to the manufacturer’s instructions. The constructed libraries were sequenced on the NovaSeq 6000 platform (Illumina).

Cell Ranger 5.0.1 was used to process the raw data from the NovaSeq 6000. Next, we analyzed the processed data using Seurat (Version 4.3.0.1) on R (Version 4.3.1). Briefly, the cells with aberrantly high or low numbers of unique molecular identifiers (UMIs) and high mitochondrial gene detection rates were removed. Gene expression levels were normalized to the total number of UMIs in each cell line. The cell cycle score was calculated using the expression levels of cell cycle-related genes to perform data scaling using cell cycle regression. The detailed process followed the developer’s instructions (https://satijalab.org/seurat/archive/v3.1/cell_cycle_vignette). Subsequently, a principal component analysis and unsupervised clustering were performed. To merge the scRNA-seq data of samples from different days, we applied the “merge” function. monocle3 (Version 1.3.3) was used for the pseudotime analysis. Pseudotime values were assigned to each cell by “learn_graph” and “order_cells” functions. Genes expressed differentially along inferred pseudotime were identified by “graph_test” function. For RNA velocity analysis, loom files were generated using Velocyto (Version 0.6). RNA velocity analysis was conducted using scVelo (Version 0.2.4) on Python (Version 3.9.5). RNA velocity was estimated by using the dynamic model. The candidates of lineage-specific driver genes were identified by “scvelo.tl.rank_velocity_genes” module. For trajectory analysis, a subset of the merged samples was processed using slingshot (Version 2.8.0) and tradeSeq (Version 1.1.4) on R. R packages dplyr (Version 1.1.3), ggplot2 (Version 3.4.3) scCustomize (Version 1.1.3) stats (Version 4.3.1) tidyr (Version 1.3.0) and Python packages anndata (Version 0.7.8), numpy (Version 1.20.3) pandas (Version 1.4.1), scanpy (Version 1.8.2) scipy (Version 1.8.0) were also used for data processing and visualization.

Statistics

Data are presented as the mean ± standard error of the mean (SEM). Significant differences were determined using two-tailed t-tests or ANOVA tests followed by Tukey–Kramer test. Microsoft Excel for Mac (Version 16.44) was used for data preprocessing and formatting.

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.

Data availability

The scRNA-seq data generated in this study have been deposited in the Gene Expression Omnibus (GEO) under accession code GSE199158. Source data are provided with this paper.

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Acknowledgements

We thank all members of the Eiraku Lab for their productive discussions. We are especially grateful to Fumi Wagai for her generous assistance in continuing the laboratory activities. We also thank the Single-cell Genome Information Analysis Core (SignAC) at WPI-ASHBi, Kyoto University, for their support with NGS experiments. This work was supported by Grant-in-Aid for Scientific Research on Innovative Areas (Ministry of Education, Culture, Sports, Science, and Technology, MEXT), Japan (16H06480 to M.E.), Grant-in-Aid for Transformative Research Areas (Ministry of Education, Culture, Sports, Science, and Technology, MEXT), Japan (23H04933 to M.E.), and Core Research for Evolutional Science and Technology (CREST, JST) (JPMJCR12W2 to M.E.).

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Authors and Affiliations

Laboratory of Developmental Systems, Institute for Life and Medical Sciences, Kyoto University, 53 Shogoin Kawaharacho, Sakyo-ku, Kyoto, 606-8507, Japan
Yusuke Seto, Kaori Takizawa & Mototsugu Eiraku
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Kyoto, 615-8510, Japan
Yusuke Seto, Ryoma Ogihara & Mototsugu Eiraku
Institute for Advanced Study of Human Biology (WPI-ASHBi), Kyoto University, Kyoto, Japan
Mototsugu Eiraku

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Yusuke Seto
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Ryoma Ogihara
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Kaori Takizawa
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Mototsugu Eiraku
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Contributions

Y.S. and M.E. conceptualized the study. Y.S. designed and conducted the experiments and performed data analysis. R.O. and K.T. performed the experiments. Y.S. wrote the manuscript. All coauthors contributed to the respective text parts, data analyses, and revised the manuscript.

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Correspondence to Yusuke Seto or Mototsugu Eiraku.

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Seto, Y., Ogihara, R., Takizawa, K. et al. In vitro induction of patterned branchial arch-like aggregate from human pluripotent stem cells. Nat Commun 15, 1351 (2024). https://doi.org/10.1038/s41467-024-45285-0

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Received: 16 May 2023
Accepted: 19 January 2024
Published: 14 February 2024
DOI: https://doi.org/10.1038/s41467-024-45285-0

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