On nearly every mammalian cell, a tiny hair-like organelle known as a primary cilium protrudes from the cell surface. These cellular appendages act as specialized antennae to survey the extracellular milieu and transmit signals into the cell that are essential for cellular homeostasis. Although primary cilia were discovered over a hundred years ago and were originally considered evolutionary remnants, interest in these organelles has increased dramatically over the last ten years due to the recognized link between primary cilia and human disease. Improper formation or function of primary cilia can result in a myriad of human diseases and genetic disorders that are collectively called ciliopathies. Due to the ubiquity of cilia, ciliopathies can affect multiple organ systems and tissues and ciliopathy patients present with a wide range of clinical features including cystic kidney disease, retinal degeneration, anosmia, obesity, polydactyly, hypogenitalism, brain malformations, and intellectual disabilities.
The pathophysiological consequences of primary cilia dysfunction highlight the important roles cilia play during development and in the normal function of most tissues. Although great progress has been made in understanding the functions of primary cilia on some cell types, primary cilia function on most cell types is still not known. This is particularly true for primary cilia on central neurons in the mammalian brain. Due to the enrichment of signaling machinery in neuronal cilia, such as G protein-coupled receptors (GPCRs) and type III adenylyl cyclase (ACIII) we hypothesize that neuronal cilia are specialized non-synaptic sensory and signaling organelles affecting neuronal function. To test this hypothesis we set out to determine the trafficking mechanisms responsible for mediating ciliary GPCR localization and determine whether proteins that localize to the ciliary membrane are functional.
We have shown that a particular protein mutated in the human ciliopathy Bardet-Biedl syndrome (BBS), Bbs5, interacts with multiple ciliary GPCRs. This finding suggests BBS proteins regulate the trafficking of GPCRs into and out of the cilium by direct interaction.
To determine whether ciliary GPCRs that are trafficked to the ciliary membrane are function, we analyzed components of the somatostatin (SST) signaling pathway. Although somatostatin receptor subtype 3 (Sstr3) was discovered to localize to neuronal cilia more than a decade ago, it has yet to be determined whether the receptor is functional within the cilium and can generate a signal. We have discovered that upon Sstr3 activation, the localization of SST signaling machinery is dynamic, suggesting neuronal cilia can sense and respond to neuromodulators by trafficking functional ciliary GPCRs and the appropriate signaling machinery to them. Furthermore, as the field of ciliary biology is ever changing, we have discovered that ciliary GPCRs can form heteromers within the mouse brain. As GPCR heteromerization can affect ligand binding properties and downstream signaling, these findings add a previously unrecognized layer of complexity to neuronal ciliary signaling.
Taken together, this work presents a novel trafficking mechanism responsible for the tightly regulated localization of ciliary GPCRs and has highlighted how neuronal cilia function as specialized signaling organelles by orchestrating signal transduction cascades important for neuronal function.