The Role of Primary Cilia in Mesenchymal Stem Cell Differentiation: A Pivotal Switch in Guiding Lineage Commitment

Abstract

Primary cilia are sensory organelles that have been shown to play a critical role in lineage commitment. It was our hypothesis that the primary cilium is necessary for chemically induced differentiation of human mesenchymal stem cells (MSC). To investigate this, polaris siRNA was used to inhibit the primary cilia and the mRNA levels of transcription factors Runx2, PPARγ were measured by RT PCR as markers of osteogenic, adipogenic and chondrogenic differentiation, respectively. MSCs with inhibited primary cilia had significantly decreased basal mRNA expression levels of all three lineages specific transcription factors indicating that primary cilia are critical in multiple differentiation pathways. Furthermore, to determine if primary cilia play a role in the differentiation potential of MSCs, progenitor cells transfected with either scrambled or polaris siRNA were cultured in osteo-inductive, chondro-inductive, or adipo-inductive media and lineage commitment was ascertained. Interestingly, within 24 h of culture, cells transfected with polaris siRNA in both osteogenic and adipogenic media lost adhesion and released from the slides; however MSCs in chondrogenic media as well as cells transfected with scrambled siRNA did not. These results suggest that the primary cilium is necessary for the normal progression of chemically induced osteogenic and adipogenic differentiation. As a control, the experiment was repeated with NIH3T3 fibroblasts and none of the effects of inhibited primary cilia were observed indicating that the loss of adhesion may be specific to MSCs. Furthermore after biochemically inducing the cells to differentiate, polaris knockdown resulted in abrogation of both Runx2 and PPARγ mRNA while SOX9 mRNA expression was significantly lower. These results suggest that primary cilia play an essential role not only in the initiation of both osteogenic and adipogenic differentiation, but also in maintaining the phenotype of differentiated cells. Interestingly, chondrogenic differentiation appeared less dependent on a functional primary cilium.

Bardet–Biedl syndrome: Is it only cilia dysfunction?

“…In addition, it was shown that while cilia are present in differentiating pre-adipocytes, cilia are lost during differentiation and mature adipocytes are non-ciliated cells [132]. Albeit transient, the presence of a primary cilium during adipocyte differentiation appears to be important since mesenchymal stem cells (MSCs) require the presence of primary cilia to differentiate toward the adipogenic lineage [133]. Moreover, Wnt and Hedgehog receptors, pathways known to regulate adipogenesis, are present in these transient cilia of differentiating pre-adipocytes [132].…”

Bicc1 is a genetic determinant of osteoblastogenesis and bone mineral density

“…In kidney epithelial cells, this complex is essential for sensing fluid flow in the developing kidney and orchestrating proper cell division (53). Mesenchymal stromal cells and mature cells of the osteoblast-osteocyte lineage also possess primary cilia (39,54), and a number of the structural proteins of the primary cilium have been shown to play a role in the regulation of BMD and osteoblast function (55,56). Additionally, inactivation of Pkd1 globally (39,57) and in osteoblasts (58) leads to osteoblast dysfunction, decreased bone formation rates, and osteopenia.…”

Craniofacial Development and Growth in Polycystic Kidney Disease

“…Pkd1 and Pkd2 may thus possess mechanical force-independent functions at embryonic stages, during growth, and into adulthood. Knockdown of the primary cilia protein, Polaris, resulted in abrogation of Runx2 (45) and since primary cilia are also present in the dental pulp, it is possible that the mechanical force-independent function of Pkd1 and Pkd2 is related to primary ciliary function.…”