Primary Cilia: The New Face of Craniofacial Research

Abstract

The primary cilium is a solitary, sensory organelle that extends from the surface of nearly every vertebrate cell, including craniofacial cells. This organelle converts chemical and physical external stimuli into intracellular signaling cascades and mediates several well-known signaling pathways simultaneously. Thus, the primary cilium is considered a cellular signaling nexus and amplifier. Primary cilia dysfunction directly results in a collection of diseases and syndromes that typically affect multiple organ systems, including the face and teeth. Despite this direct connection, primary cilia are largely unexplored in craniofacial research. In this review, I briefly summarize craniofacial abnormalities tied to the primary cilium and examine the existing information on primary cilia in craniofacial development and repair. I close with a discussion on preliminary studies that motivate future areas of exploration that are further supported by studies performed in long bone and kidney cells.

Keywords: Hh signaling; Wnt signaling; calcium signaling; ciliopathies; cleft lip; cleft palate; dental pulp stem cells; neurons; odontoblasts; primary cilia.

Figure 1

Primary cilium structure and intraflagellar transport (IFT) components. The primary cilium is composed of an anchoring basal body and an axoneme that extends from the cell surface. The axoneme is encapsulated by a ciliary membrane that is distinct from but continuous with the plasma membrane. Signaling proteins, such as ion channels and receptors, are especially concentrated near the axoneme base to facilitate signaling amplification. The axoneme structure is maintained by IFT and its components including kinesin-II, dynein-II, IFT-A, and IFT-B. This figure was created with BioRender.com (accessed on 10 November 2022).

Figure 2

Dental cells known to contain primary cilia. Primary cilia are reported in cells that participate in tooth development (A–C) and make up the structures associated with fully formed teeth (D). (A) Cilia are detected as early as the initiation stage. (B) Cap stage: dental lamina (DL), primary enamel knot (pEK), dental papilla (DPa), dental follicle (DF). (C) Bell stage with magnified region where dentin (Dn) and enamel (E) are initially deposited: dental stalk (DS) and secondary enamel knot (sEK). (D) Fully developed and erupted molars anchored to alveolar bone (AB): dental pulp (DPu) and periodontal ligament (PDL). (E) Legend for the cells depicted in A–D: dental pulp stem cell (DPSC) and bone marrow stromal cell (BMSC). This figure was created with BioRender.com (accessed on 16 November 2022).

Figure 3

Primary cilium components associated with proteins linked to craniofacial abnormalities. Primary cilia function can be disrupted by altering a variety of components within the ciliary complex. Loss of KIF3A renders the motor protein kinesin-II non-functional. Removal of IFT proteins such as IFT20, 80, 88, and 140 alters the activity of IFT-A and IFT-B complexes, as well as their ability to bind motor proteins. Protein trafficking and localization are disrupted with mutations in proteins located in the ciliary membrane (ARL13B) or the transition zone (CEP20 and MSK1). The axoneme is unable to form normally when the basal body is destabilized from loss of proteins such as OFD1. This figure was created with BioRender.com (accessed on 14 November 2022).

Figure 4

Summary of proposed future directions for dental primary cilia research. (A) Primary cilia may be important for odontoblasts (yellow) to sense physical stimuli from compression of the tooth and/or fluid shear (black arrows) through the dentinal tubules. (B) Fluid shear through the dentinal tubules may elicit an intracellular calcium response (green arrow) in odontoblasts that is mediated by the primary cilium. The axoneme bends in the direction of the fluid, causing calcium ion channels (yellow) to open (red) so that calcium (blue spheres) enters the ciliary complex and binds to proteins, triggering intracellular signaling cascades (red arrows). (C) During inflammation and injury, sensory nerves (purple) secrete factors that regulate dental stem cell (green) apoptosis, proliferation, and odontogenic differentiation in mechanisms that are potentially mediated by dental stem cell primary cilia. (D) Components associated with several signaling pathways (represented by different colors) are regulated simultaneously by the primary cilium in both chemo- and mechanotransduction. The individual contribution of each pathway is synchronized by the primary cilium to produce a coordinated intracellular response (brown arrow). This figure was created with BioRender.com (accessed on 15 November 2022).