Canonical and noncanonical intraflagellar transport regulates craniofacial skeletal development

Abstract

The primary cilium is a cellular organelle that coordinates signaling pathways critical for cell proliferation, differentiation, survival, and homeostasis. Intraflagellar transport (IFT) plays a pivotal role in assembling primary cilia. Disruption and/or dysfunction of IFT components can cause multiple diseases, including skeletal dysplasia. However, the mechanism by which IFT regulates skeletogenesis remains elusive. Here, we show that a neural crest-specific deletion of intraflagellar transport 20 (Ift20) in mice compromises ciliogenesis and intracellular transport of collagen, which leads to osteopenia in the facial region. Whereas platelet-derived growth factor receptor alpha (PDGFRα) was present on the surface of primary cilia in wildtype osteoblasts, disruption of Ift20 down-regulated PDGFRα production, which caused suppression of PDGF-Akt signaling, resulting in decreased osteogenic proliferation and increased cell death. Although osteogenic differentiation in cranial neural crest (CNC)-derived cells occurred normally in Ift20-mutant cells, the process of mineralization was severely attenuated due to delayed secretion of type I collagen. In control osteoblasts, procollagen was easily transported from the endoplasmic reticulum (ER) to the Golgi apparatus. By contrast, despite having similar levels of collagen type 1 alpha 1 (Col1a1) expression, Ift20 mutants did not secrete procollagen because of dysfunctional ER-to-Golgi trafficking. These data suggest that in the multipotent stem cells of CNCs, IFT20 is indispensable for regulating not only ciliogenesis but also collagen intracellular trafficking. Our study introduces a unique perspective on the canonical and noncanonical functions of IFT20 in craniofacial skeletal development.cranial neural crest cells | intracellular trafficking | intraflagellar transport | PDGF signaling | primary cilia

Genes and molecular pathways underpinning ciliopathies

“…It will be interesting to determine whether other cell type-specific centrosome-associated functions represent divergent functions for the ciliary machinery. IFT20 may be particularly versatile in its functions as, in addition to its roles in trafficking cargo within the cilium and to the immunological synapse, it contributes to intracellular transport of collagen 166 .…”

“…For example, the role of IFT20 in collagen trafficking, which was discovered in a mouse model 166 of a craniofacial skeletal development disorder, may be ultimately linked to an extraciliary disorder.…”

Primary Cilia and Coordination of Receptor Tyrosine Kinase (RTK) and Transforming Growth Factor β (TGF-β) Signaling

“…In line with this result, analysis of PDGFRa mRNA and protein levels showed that PDGFRa is highly upregulated during serum deprivation in cultured cells, concomitantly with formation of the primary cilium (Lih et al 1996;Schneider et al 2005). Subsequent studies have confirmed cilia-specific localization of PDGFRa in a range of additional cell types, including rat astrocytes and neuroblasts (Danilov et al 2009), mouse heart ventricular cells (Gerhardt et al 2013), human embryonic stem cells (Awan et al 2010), ovarian surface epithelial cells (Egeberg et al 2012), and mouse osteoblasts (Noda et al 2016). Of note, in some ciliated cell types, such as rat oligodendrocytes (Falcon-Urrutia et al 2015) and mouse heart atrial cells (Gerhardt et al 2013), PDGFRa seems to be conspicuously absent from the organelle, whereas in retinal pigment epithelial (RPE) cells, which are commonly used in ciliary studies, PDGFRa seems hardly to be expressed at all (Lei et al 2011;Nielsen et al 2015).…”

Regional difference in microRNA regulation in the skull vault

“…The contributions of neural crest cells to mouse skull vault have been studies recently. 5,12,36,37 With the initiation of neural crests patterning to osteogenic lineage tracking, different transcriptional factors were found critical, 12,38,39 such as Runx2, Osterix, Msx2, and Dlx5. Msx2 drives CNC cells differentiation, and deletion of Msx2 in neural crest-specific manner results in the defects in frontal bone.…”