ABSTRACT: Purpose: We have succeeded in the generation and long-term expansion of SOX9-expressing CD271+PDGFRa+CD73+ chondrogenic ectomesenchymal cells from the PAX3/SOX10/FOXD3-expressing MIXL1-CD271hiPDGFRaloCD73- neural crest-like progeny of human pluripotent stem cells in a chemically defined medium supplemented with Nodal/Activin/TGFb inhibitor (SB) and FGF2 (hereafter called FSB). When “primed” with TGFb, such cells efficiently formed translucent cartilage particles, which were completely mineralized in 12 weeks in immunocompromized mice. Under the FSB condition, ectomesenchymal cells were expandable without loss of chondrogenic potential for at least 16 passages, maintained normal karyotype for at least 10 passages, which any conditions deviated from it (e.g. FGF2 alone or SB alone) failed to support. In order to address the molecular basis of such effects of FSB, a series of RNA-seq experiments were carried out. Methods: We generated and compared the transcriptome profiles of human ectomesenchymal cells expanded under FSB with those cultured under FSB first then under FGF2 alone (F). As a control, we also generated transcriptome of ectomesenchymal cells expanded from the begining under F conditions. RNA-sequencing libraries were prepared using a SureSelect Strand Specific RNA Library Preparation kit (Agilent technologies, Santa Clara, CA). Sequencing was performed on an Illumina HiSeq 1500 using a TruSeq Rapid SBS kit (Illumina, San Diego, CA) in a 50-base single-end mode. Sequenced reads were mapped against the human reference genome (GRCh37), using TopHat v2.0.12 (http://ccb.jhu.edu/software/tophat/index.shtml). Expression levels were calculated as fragments per kilobase of exon per million mapped fragments (FPKMs) using Cufflinks v2.1.1 (http://cole-trapnell-lab.github.io/cufflinks). Results: Ectomesenchymal cells maintained under FSB conditions preferentially expressed genes representing embryonic progenitors (SOX4/12, LIN28A/B, MYCN), cranial mesenchymes (ALX1/3/4) and chondroprogenitors (SOX9, COL2a1) of the neural crest origin (SOX8/9, NGFR, NES). In contrast, those cultured under FSB then F, still expressed SOX4/11/12, but lost LIN28, MYCN, ALX1/3/4, NGFR, COL2a1 expression. Interestingl it enhances expresion ofTGFβ-inducible genes such as THBS1/2 and DCN and osteogenesis-related genes such as COL1a1/2 and RUNX1/2. Conclusions: The CD271+CD73+ ectomesenchymal cells accumulated under FSB conditions possess an mRNA profile of proliferating primitive neural crest/ectomesenchymal cells, although they lacked SOX10 expression, which is critical for neural and melanocytic lineage commitment. Transition from FSB to F conditions supressed the proliferation-related genes expression and enhanced the ossification/mineralization, vasculogenesis/angiogenesis, and cardiac myogenesis-related gene expression. Thus, suppression of TGFβ signaling by SB does not seem to freeze the developmental stage of the hPSC-derived neural crest during expansion. Such suppression may instead simply support the high proliferative potential of the cells as well as the expression of SOX9 (and COL2a1), and thereby maintain chondrogenic activity. SOX9 expression initiated at the specification and pre-migratory stages is transient in trunk neural crest but persists in cranial neural crest. The chondrogenic CD271+CD73+ ectomesenchymal cells that maintain SOX9 transcription and translation may therefore represent proliferating cranial neural crest, with a slight commitment to non-neural lineages.
