Targeting of vasoactive intestinal peptide receptor 2, VPAC2, a secretin family G-protein coupled receptor, to primary cilia

Golgi Dysfunctions in Ciliopathies

The Golgi apparatus (GA) is essential for intracellular sorting, trafficking and the targeting of proteins to specific cellular compartments. Anatomically, the GA spreads all over the cell but is also particularly enriched close to the base of the primary cilium. This peculiar organelle protrudes at the surface of almost all cells and fulfills many cellular functions, in particular during development, when a dysfunction of the primary cilium can lead to disorders called ciliopathies. While ciliopathies caused by loss of ciliated proteins have been extensively documented, several studies suggest that alterations of GA and GA-associated proteins can also affect ciliogenesis. Here, we aim to discuss how the loss-of-function of genes coding these proteins induces ciliary defects and results in ciliopathies.

Primary Cilia Signaling Shapes the Development of Interneuronal Connectivity

Appropriate growth and synaptic integration of GABAergic inhibitory interneurons are essential for functional neural circuits in the brain. Here, we demonstrate that disruption of primary cilia function following the selective loss of ciliary GTPase Arl13b in interneurons impairs interneuronal morphology and synaptic connectivity, leading to altered excitatory/inhibitory activity balance. The altered morphology and connectivity of cilia mutant interneurons and the functional deficits are rescued by either chemogenetic activation of ciliary G-protein-coupled receptor (GPCR) signaling or the selective induction of Sstr3, a ciliary GPCR, in Arl13b-deficient cilia. Our results thus define a specific requirement for primary cilia-mediated GPCR signaling in interneuronal connectivity and inhibitory circuit formation.

Recruitment of β-Arrestin into Neuronal Cilia Modulates Somatostatin Receptor Subtype 3 Ciliary Localization

Primary cilia are essential sensory and signaling organelles present on nearly every mammalian cell type. Defects in primary cilia underlie a class of human diseases collectively termed ciliopathies. Primary cilia are restricted subcellular compartments, and specialized mechanisms coordinate the localization of proteins to cilia. Moreover, trafficking of proteins into and out of cilia is required for proper ciliary function, and this process is disrupted in ciliopathies. The somatostatin receptor subtype 3 (Sstr3) is selectively targeted to primary cilia on neurons in the mammalian brain and is implicated in learning and memory. Here, we show that Sstr3 localization to cilia is dynamic and decreases in response to somatostatin treatment. We further show that somatostatin treatment stimulates β-arrestin recruitment into Sstr3-positive cilia and this recruitment can be blocked by mutations in Sstr3 that impact agonist binding or phosphorylation. Importantly, somatostatin treatment fails to decrease Sstr3 ciliary localization in neurons lacking β-arrestin 2. Together, our results implicate β-arrestin in the modulation of Sstr3 ciliary localization and further suggest a role for β-arrestin in the mediation of Sstr3 ciliary signaling.

Rhodopsin Trafficking and Mistrafficking: Signals, Molecular Components, and Mechanisms

Rhodopsin is a seven-transmembrane G protein-coupled receptor (GPCR) and is the main component of the photoreceptor outer segment (OS), a ciliary compartment essential for vision. Because the OSs are incapable of protein synthesis, rhodopsin must first be synthesized in the inner segments (ISs) and subsequently trafficked across the connecting cilia to the OSs where it participates in the phototransduction cascade. Rapid turnover of the OS necessitates a high rate of synthesis and efficient trafficking of rhodopsin to the cilia. This cilia-targeting mechanism is shared among other ciliary-localized GPCRs. In this review, we will discuss the process of rhodopsin trafficking from the IS to the OS beginning with the trafficking signals present on the protein. Starting from the endoplasmic reticulum and the Golgi apparatus within the IS, we will cover the molecular components assisting the biogenesis and the proper sorting. We will also review the confirmed binding and interacting partners that help target rhodopsin toward the connecting cilium as well as the cilia-localized components which direct proteins into the proper compartments of the OS. While rhodopsin is the most critical and abundant component of the photoreceptor OS, mutations in the rhodopsin gene commonly lead to its mislocalization within the photoreceptors. In addition to covering the trafficking patterns of rhodopsin, we will also review some of the most common rhodopsin mutants which cause mistrafficking and subsequent death of photoreceptors. Toward the goal of understanding the pathogenesis, three major mechanisms of aberrant trafficking as well as putative mechanisms of photoreceptor degeneration will be discussed.

Nephrocystin proteins NPHP5 and Cep290 regulate BBSome integrity, ciliary trafficking and cargo delivery

Proper functioning of cilia, hair-like structures responsible for sensation and locomotion, requires nephrocystin-5 (NPHP5) and a multi-subunit complex called the Bardet-Biedl syndrome (BBS)ome, but their precise relationship is not understood. The BBSome is involved in the trafficking of membrane cargos to cilia. While it is known that a loss of any single subunit prevents ciliary trafficking of the BBSome and its cargos, the mechanisms underlying ciliary entry of this complex are not well characterized. Here, we report that a transition zone protein NPHP5 contains two separate BBS-binding sites and interacts with the BBSome to mediate its integrity. Depletion of NPHP5, or expression of NPHP5 mutant missing one binding site, specifically leads to dissociation of BBS2 and BBS5 from the BBSome and loss of ciliary BBS2 and BBS5 without compromising the ability of the other subunits to traffic into cilia. Depletion of Cep290, another transition zone protein that directly binds to NPHP5, causes additional dissociation of BBS8 and loss of ciliary BBS8. Furthermore, delivery of BBSome cargos, smoothened, VPAC2 and Rab8a, to the ciliary compartment is completely disabled in the absence of single BBS subunits, but is selectively impaired in the absence of NPHP5 or Cep290. These findings define a new role of NPHP5 and Cep290 in controlling integrity and ciliary trafficking of the BBSome, which in turn impinge on the delivery of ciliary cargo.

Neuropeptide Y protects cerebral cortical neurons by regulating microglial immune function

Neuropeptide Y has been shown to inhibit the immunological activity of reactive microglia in the rat cerebral cortex, to reduce N-methyl-D-aspartate current (I_NMDA) in cortical neurons, and protect neurons. In this study, after primary cultured microglia from the cerebral cortex of rats were treated with lipopolysaccharide, interleukin-1β and tumor necrosis factor-α levels in the cell culture medium increased, and mRNA expression of these cytokines also increased. After primary cultured cortical neurons were incubated with the lipopolysaccharide-treated microglial conditioned medium, peak I_NMDA in neurons increased. These effects of lipopolysaccharide were suppressed by neuropeptide Y. After addition of the neuropeptide Y Y1 receptor antagonist BIBP3226, the effects of neuropeptide Y completely disappeared. These results suggest that neuropeptide Y prevents excessive production of interleukin-1β and tumor necrosis factor-α by inhibiting microglial reactivity. This reduces I_NMDA in rat cortical neurons, preventing excitotoxicity, thereby protecting neurons.

G-protein-coupled receptors and localized signaling in the primary cilium during ventral neural tube patterning

The primary cilium is critical in sonic hedgehog (Shh)-dependent ventral patterning of the vertebrate neural tube. Most mutants that cause disruption of the cilium result in decreased Shh signaling in the neural tube. In contrast, mutations in the intraflagellar complex A (IFT-A) and the tubby family protein, Tulp3, result in increased Shh signaling in the neural tube. Proteomic analysis of Tulp3-binding proteins first pointed to the role of the IFT-A complex in trafficking Tulp3 into the cilia. Tulp3 directs trafficking of rhodopsin family G-protein-coupled receptors (GPCRs) to the cilia, suggesting the role of a GPCR in mediating the paradoxical effects of the Tulp3/IFT-A complex in causing increased Shh signaling. Gpr161 has recently been identified as a Tulp3/IFT-A-regulated GPCR that localizes to the primary cilium. A null knock-out mouse model of Gpr161 phenocopies Tulp3 and IFT-A mutants, and causes increased Shh signaling throughout the neural tube. In the absence of Shh, the bifunctional Gli transcription factors are proteolytically processed into repressor forms in a protein kinase A (PKA) -dependent and cilium-dependent manner. Gpr161 activity results in increased cAMP levels in a Gαs -coupled manner, and determines processing of Gli3. Shh signaling also results in removal of Gpr161 from the cilia, suggesting that Gpr161 functions in a positive feedback loop in the Shh pathway. As PKA-null and Gαs mutant embryos also exhibit increased Shh signaling in the neural tube, Gpr161 is a strong candidate for a GPCR that regulates ciliary cAMP levels, and activates PKA in close proximity to the cilia.

G-protein-coupled receptors, Hedgehog signaling and primary cilia

The Hedgehog (Hh) pathway has become an important model to study the cell biology of primary cilia, and reciprocally, the study of ciliary processes provides an opportunity to solve longstanding mysteries in the mechanism of vertebrate Hh signal transduction. The cilium is emerging as an unique compartment for G-protein-coupled receptor (GPCR) signaling in many systems. Two members of the GPCR family, Smoothened and Gpr161, play important roles in the Hh pathway. We review the current understanding of how these proteins may function to regulate Hh signaling and also highlight some of the critical unanswered questions being tackled by the field. Uncovering GPCR-regulated mechanisms important in Hh signaling may provide therapeutic strategies against the Hh pathway that plays important roles in development, regeneration and cancer.

Lack of dopaminergic inputs elongates the primary cilia of striatal neurons

In the rodent brain, certain G protein-coupled receptors and adenylyl cyclase type 3 are known to localize to the neuronal primary cilium, a primitive sensory organelle protruding singly from almost all neurons. A recent chemical screening study demonstrated that many compounds targeting dopamine receptors regulate the assembly of Chlamydomonas reinhardtii flagella, structures which are analogous to vertebrate cilia. Here we investigated the effects of dopaminergic inputs loss on the architecture of neuronal primary cilia in the rodent striatum, a brain region that receives major dopaminergic projections from the midbrain. We first analyzed the lengths of neuronal cilia in the dorsolateral striatum of hemi-parkinsonian rats with unilateral lesions of the nigrostriatal dopamine pathway. In these rats, the striatal neuronal cilia were significantly longer on the lesioned side than on the non-lesioned side. In mice, the repeated injection of reserpine, a dopamine-depleting agent, elongated neuronal cilia in the striatum. The combined administration of agonists for dopamine receptor type 2 (D2) with reserpine attenuated the elongation of striatal neuronal cilia. Repeated treatment with an antagonist of D2, but not of dopamine receptor type 1 (D1), elongated the striatal neuronal cilia. In addition, D2-null mice displayed longer neuronal cilia in the striatum compared to wild-type controls. Reserpine treatment elongated the striatal neuronal cilia in D1-null mice but not in D2-null mice. Repeated treatment with a D2 agonist suppressed the elongation of striatal neuronal cilia on the lesioned side of hemi-parkinsonian rats. These results suggest that the elongation of striatal neuronal cilia following the lack of dopaminergic inputs is attributable to the absence of dopaminergic transmission via D2 receptors. Our results provide the first evidence that the length of neuronal cilia can be modified by the lack of a neurotransmitter's input.

Influences of Primary Cilia on Cortical Morphogenesis and Neuronal Subtype Maturation

Recognition that virtually every neuronal progenitor cell and neuron in the cerebral cortex is ciliated has triggered intense interest in neuronal cilia function. Here, we review recent studies that suggest the primary cilia of cortical progenitor cells are required for establishing and maintaining the organization within pools of proliferative cells. In addition, signaling via primary cilia differentially influence the migration and differentiation of excitatory and inhibitory neurons in the developing cortex. Specifically, the primary cilia of excitatory neurons appear to play a significant role in regulating the post-migratory differentiation of these neurons whereas cilia of inhibitory neurons appear to be required for the proper migration and positioning of those cells in cortex. Given the recently discovered functions of cilia in proliferation, neuronal migration, and differentiation, it is likely that further studies of cilia signaling will improve our understanding of how these basic developmental processes are regulated and may provide insight into how mutations in specific cilia genes linked to ciliopathies lead to the many neurological deficits associated with these diseases.

Agonist-induced GPCR shedding from the ciliary surface is dependent on ESCRT-III and VPS4

BACKGROUND

Membrane trafficking of G protein-coupled receptors (GPCRs) is crucial for temporal and spatial control of cell-surface GPCR signaling. Receptor internalization is a well-documented method cells use for regulating a wide variety of GPCRs following their exposure to agonists.

RESULTS

We report that, upon agonist stimulation, a GPCR called vasoactive intestinal peptide receptor 2 (VPAC2) is shed, rather than being internalized, in vitro and in vivo, from the membrane of primary cilia--solitary hair-like organelles that project from the cell surface. VPAC2 is released into the extracellular milieu in the form of ciliary ectosomes that are devoid of exosome markers. The agonist-induced VPAC2 shedding is selective, as shown by the fact that other ciliary membrane proteins including two ciliary GPCRs are not shed with VPAC2. VPAC2 ectosome shedding is dependent on several components of endosomal sorting complexes required for transport (ESCRT), including a subset of ESCRT-III, VPS4, and LIP5. Agonist-stimulated VPAC2 is important for ciliary-ectosome generation because it allows VPS4 and LIP5 to transiently accumulate in primary cilia. Shedding of VPAC2 from the ciliary surface results in termination of intracellular VPAC2 signaling.

CONCLUSIONS

Agonist-induced GPCR shedding from the ciliary surface may represent an additional mode of GPCR trafficking and signal regulation.

Cilia and coordination of signaling networks during heart development

Primary cilia are unique sensory organelles that coordinate a wide variety of different signaling pathways to control cellular processes during development and in tissue homeostasis. Defects in function or assembly of these antenna-like structures are therefore associated with a broad range of developmental disorders and diseases called ciliopathies. Recent studies have indicated a major role of different populations of cilia, including nodal and cardiac primary cilia, in coordinating heart development, and defects in these cilia are associated with congenital heart disease. Here, we present an overview of the role of nodal and cardiac primary cilia in heart development.

The Roles of Primary cilia in Polycystic Kidney Disease

Autosomal dominant polycystic kidney disease (ADPKD) is an inherited genetic disorder that results in progressive renal cyst formation with ultimate loss of renal function and other systemic disorders. These systemic disorders include abnormalities in cardiovascular, portal, pancreatic and gastrointestinal systems. ADPKD is considered to be among the ciliopathy diseases due to the association with abnormal primary cilia function. In order to understand the full course of primary cilia and its association with ADPKD, the structure, functions and role of primary cilia have been meticulously investigated. As a result, the focus on primary cilia has emerged to support the vital roles of primary cilia in ADPKD. The primary cilia have been shown to have not only a mechanosensory function but also a chemosensory function. Both structural and functional defects in primary cilia result in cystic kidney disease and vascular hypertension. Thus, the mechanosenory and chemosensory functions will be analyzed in regards to ADPKD.