Iben Rønn Veland:
Primary cilia are microtubule-based, sensory organelles that emerge from the centrosomal mother centriole to project from the surface of most quiescent cells in the human body. Ciliary entry is a tightly controlled process, involving diffusion barriers and gating complexes that maintain a unique composition of receptors and signal components in the cilium to regulate cellular processes such as transcriptional control or cytoskeletal reorganization. This dissertation focuses on selected signaling systems regulated by the primary cilium, including the PDGFRα, TGFβ and Wnt pathways, and how this controls directional cell migration as a physiological response.
The ciliary pocket is a membrane invagination with elevated activity of clathrin-dependent endocytosis (CDE). In paper I, we show that the primary cilium regulates TGF-β signaling and the ciliary pocket is a compartment for CDE-dependent regulation of signal transduction. Upon ligand-binding and activation in the cilium, TGFβ receptors accumulate and are internalized at the ciliary base together with Smad2/3 transcription factors that are phosphorylated here and translocated to the nucleus for target gene expression. These processes depend on formation of the primary cilium and CDE at the pocket region.
The ciliary protein Inversin functions as a molecular switch between canonical and non-canonical Wnt signaling. In paper II, we show that Inversin and the primary cilium control Wnt signaling and are required for polarization and cell migration. A number of central Wnt components localize to the fibroblast primary cilium, including the Wnt5a-receptor, Fzd3, and Dvl proteins. Inversin-deficient MEFs have an elevated expression of canonical Wnt-associated genes and proteins, in addition to dysregulation of components in non-canonical Wnt signaling and cytoskeletal organization. Further, cell migration and polarization in are impaired in Invs MEFs.
In two-dimensional cell migration, the centrosome is positioned between the nucleus and the leading edge with the primary cilium directed towards the direction of migration. PDGFRα is activated in the primary cilium upon stimulation with PDGF-AA, a chemotactic agent. In paper IV, we use methods described in paper III to show that the primary cilium controls directional cell migration in wound healing in PDGF-AA-mediated chemotaxis. In vitro and in vivo wound closure is impaired by defective cilia formation, which leads to uncontrolled cell movements.
Together, the results obtained from my PhD studies reflect the high level of complexity within signaling systems regulated by the primary cilium that control cellular processes during embryonic development and in tissue homeostasis. As such, this dissertation can contribute to expanding the current understanding of the genetic mechanisms underlying ciliopathies.