Primary cilia-dependent signaling is involved in regulating mesenchymal stem cell proliferation and pluripotency maintenance

Using a large-scale quantitative mesenchymal stem cells (MSCs) membrane proteomics analysis, we identified a large group of ciliary proteins in the MSCs membrane fraction, which prompted us to examine the cilia, intricate organelles that were originally discovered approximately 100 years ago. Here we characterize their primary structure and function in MSCs. We first characterized the primary cilia on undifferentiated human MSCs by immunostaining and verified these observation with scanning and 3D electronic microscopy. To investigate the function of the primary cilia of the MSCs we induced loss of function by means of siRNA knockdown (targeted to two known ciliary proteins: IFT172 and KIF3A). After either of these two proteins was knocked down by the respective siRNA, the MSCs showed fewer and shortened primary cilia. The MSCs proliferation assays showed increased cell proliferative activity under confluent conditions after the siRNA knockdown of IFT172 or KIF3A; among these MSCs, the proportion in S phase was increased in the IFT172 siRNA knockdown group. The expression of stem cell markers on the MSCs, namely, Oct4, Nanog and Sox2, were downregulated after the siRNA-induced knockdown of IFT172 or KIF3A, and the gene expression upregulation of ectoderm lineage markers was notable. Furthermore, manipulation of the primary cilia-dependent Shh pathway, using the Shh activator SAG (smoothened agonist), upregulated the gene expression of pluripotency markers, including Nanog and Oct4, and transcriptional target genes in the Shh pathway. This study confirms that MSCs have primary cilia and provides evidence that primary cilia-dependent signaling pathways play functional roles in MSCs proliferation and stemness maintenance.