The Primary Cilium as the Cell's Antenna: Signaling at a Sensory Organelle

Abstract

Almost every vertebrate cell has a specialized cell surface projection called a primary cilium. Although these structures were first described more than a century ago, the full scope of their functions remains poorly understood. Here, we review emerging evidence that in addition to their well-established roles in sight, smell, and mechanosensation, primary cilia are key participants in intercellular signaling. This new appreciation of primary cilia as cellular antennae that sense a wide variety of signals could help explain why ciliary defects underlie such a wide range of human disorders, including retinal degeneration, polycystic kidney disease, Bardet-Biedl syndrome, and neural tube defects.

Primary cilia sensitize endothelial cells for fluid shear stress

“…Alternatively, nodal flow has been demonstrated to deliver signalling proteins, such as sonic hedgehog or fibroblast growth factor, ensheathed by nodal vesicular parcels to the left side of the node where they induce downstream signalling (Hirokawa et al, 2006;Okada et al, 1999;Tanaka et al, 2005). Intraflagellar transport along the ciliary axoneme involving the Patched, Smoothened, and Gli transcription factors was recently suggested to be essential for hedgehog signalling (Huangfu et al, 2003;Singla and Reiter, 2006), bridging part of the gap between both theories about the role of primary cilia in determining left-right asymmetry. The mechanism inducing laterality is very conserved among species, as situs inversus due to abnormal laterality gene expression is present in distant species like eukaryotes and snails .…”

“…Especially the interaction complex involving CD31/PECAM1, VECadherin, and VEGFR2/KDR/FLK1 has been well documented (Tzima et al, 2005). Besides a molecular sensor complex, an ultrastructural adaptation for mechanosensing, called a primary cilium, has been described (Singla and Reiter, 2006). It is a membrane extension with a core of 9 doublets of microtubules that are connected to the basal body of the cell.…”

Three types of cilia including a novel 9+4 axoneme on the notochordal plate of the rabbit embryo

“…These ancient and nearly ubiquitous organelles are found from unicellular algae to protozoa and animals with a wide diversity of functions. In epithelia of the lung, for example, multiciliated cells use synchronized ciliary beating to move mucus over the surface; single flagella of sperm or protozoan cells may propel the whole cell body through surrounding liquid, and immotile sensory cilia serve as cellular antennae detecting extracellular signals (Snell et al, 2004;Davis et al, 2006;Marshall and Nonaka, 2006;Singla and Reiter, 2006).…”

Epiphyseal growth plate and secondary peripheral chondrosarcoma: the neighbours matter

“…The primary cilia function as the signalling 'antennae' of the cells that receives and transduces mechanical and chemical signals from the neighbouring cells and the ECM (Figure 1) [31]. During development, the positioning of the primary cilium of the hair cells in the sensory epithelium leads and predicts the polarity of each stereociliary bundle, supporting the hypothesis that primary cilia direct the polarization of hair cells [32,33].…”