Cilia, tubby mice, and obesity
© Mukhopadhyay and Jackson; licensee BioMed Central Ltd. 2013
Received: 6 November 2012
Accepted: 7 November 2012
Published: 3 January 2013
Primary cilia have been previously linked to the central regulation of satiety. Thetubby mouse is characterized by maturity-onset obesity and blindness. Arecent paper demonstrates molecular defects in trafficking of ciliary GPCRs in thecentral neurons of tubby mice, underscoring the role of ciliary signaling inthe pathogenesis of this monogenic obesity syndrome.
Please see related Research article by Li et al.,http://www.ciliajournal.com/content/1/1/21
Most neurons in the vertebrate nervous system elaborate primary cilia. Historically,neuronal primary cilia were first identified in neuroepithelial progenitor cellsprojecting into the neural tube lumen. Later on, they were described to be broadlypresent both in neurons and glia . The primary cilia function as sensory antennae in a wide variety of cells.Cilia-localized receptors, which include certain G protein-coupled receptors (GPCRs),and their downstream effectors, determine the sensory modality of cells in specificcontexts, especially during vertebrate photoreception or olfaction and for responding tomorphogens, such as Sonic hedgehog (Shh). Although we have come to appreciate thefunction of the primary cilium in other tissues and organisms, the functional roles ofthis ubiquitous neuronal organelle in integrating neuroendocrine signals have remainedenigmatic. Diseases resulting from disruption of primary cilia and the associated basalbody complex, called ciliopathies, often have strong neurological components,emphasizing the role of this cellular compartment in neural development . Interestingly, aside from the strong neurodevelopmental phenotypes,progressive obesity often affects patients with certain ciliopathies such as theBardet-Biedel Syndrome (BBS) and Alström Syndrome . Notably, conditional knockout of components of the cilia in the micehypothalamus results in hyperphagia-induced obesity and underscores the role of ciliarysignaling in the central regulation of satiety . Thus, it is imperative to achieve a better understanding of the ciliarysignaling pathways in central satiety networks, which could lead to novel ways fortreating the global obesity pandemic.
The tubby mouse was initially identified as a spontaneous maturity-onsetobesity syndrome , and positional cloning strategies in the 1990s mapped the causative mutationto a novel gene of unknown function called Tub[6, 7]. In nematodes, tub-1, the canonical Tub homolog wasidentified in an RNAi screen for fat storage defects , and was found to be expressed in the ciliated neurons , highlighting the role of neuroendocrine signals in maintenance of systemicfat homeostasis even in these distant evolutionary relatives. Thanks to a recent paperfrom Sun et al.  the tubby mouse can now be added to the growing list of monogenicobesity syndromes with a strong ciliary functional component in the central nervoussystem . The authors demonstrate molecular defects in ciliary GPCR signaling in thetubby mice, suggesting the importance of ciliary GPCR trafficking in centralneurons implicated in satiety circuits.
The authors show that in the tubby mice, the primary cilia in the neurons showno obvious structural defects. However, two ciliary GPCRs, melanin-concentrating hormonereceptor 1 (Mchr1) and somatostatin receptor subtype 3 (Sstr3), known to localize todistinct regions of the brain [11, 12], are strongly prevented from trafficking to the primary cilia. This phenotypeis strongly reminiscent of a previous study showing defective ciliary targeting of thesereceptors in BBS mice . Similar to the BBS mice, the tubby mice also display retinaldegeneration, and a defect in trafficking of rhodopsin to the outer segment of thephotoreceptor, an extension of the connecting cilia in these cells. However, indistinction from the BBS mice that have defective olfactory cilia and are anosmic , tubby mice do not show defects in either the structure of thesespecialized cilia or in localization of olfactory GPCRs. This difference could bebecause Tub is not expressed and does not play a major role in these specificneurons, or because the presence of other tubby family homologs (such as Tulp3)compensates for the loss of Tub activity. The authors also detect the defects in ciliaryGPCR trafficking well in advance of the development of obesity and retinal degeneration,implying that these trafficking defects could be causative for the development of thesephenotypes.
How does Tubby affect ciliary GPCR trafficking? The Tub gene is the foundingmember of a family of proteins , characterized by a C-terminal tubby domain, which is highly specific forbinding to 4,5 phosphoinsositides (PIP2) . This domain is likely to participate in binding to specific membranecompartments, which for Tubby may be the ciliary membrane. Some of the tubby familymembers (including Tub, Tulp2 and Tulp3) also have a signature motif in the divergentN-terminus that binds to the core subunits of the ciliary intraflagellar transportcomplex-A (IFT-A) . Tulp3 mutant mice are embryonic lethal by mid-gestation , but previous in vitro studies with heterologous cultured ciliatedcell lines suggested that both Tulp3 and IFT-A core subcomplex direct GPCR traffickingto the cilia . Careful mutational analysis of both the IFT-A binding N-terminal andPIP2-binding C-terminal domains suggest that both the IFT-A- and membranephosphoinositide-binding properties of TULP3 are necessary for ciliary GPCRlocalization. TULP3 thus bridges the IFT-A complex to the membrane compartment in gatingciliary GPCR trafficking, although the specific mechanism of ciliary GPCR recruitmentremains to be determined. Most importantly, in the context of neuronal ciliary GPCRtrafficking, the Tulp3 N-terminal fragment can act as a dominant negative reagent,preventing GPCR trafficking in cultured hippocampal neurons . Tub also shares the IFT-A binding motif with Tulp3, and binds to the IFT-Acomplex , although possibly less efficiently. Thus, similar to Tulp3, Tub could bedirecting ciliary GPCR trafficking through its simultaneous binding to the IFT-Acomplex, and membrane phosphoinositides. Presumably, higher levels of Tub in the braincould compensate for the lower binding or weaker affinity of Tub for the IFT-A complex.According to the Allen Brain Atlas, hypothalamic Tub transcript levels areabout 26 times that of Tulp3. However, Tub/IFT-A might also require additionalfactors. Besides, the dominant negative IFT-A-binding N-terminal fragment of Tulp3 wouldbe expected to inhibit both Tulp3 and Tub binding to the IFT-A complex in these neurons,effectively shutting down complementary effects of these proteins in trafficking ciliaryGPCRs. Thus, based on the spatial and temporal expression of these specific tubby familyproteins in different tissues, and their affinity for the IFT-A complex, we might expectto observe a differential effect in their relative capacities for gating ciliary GPCRs.These differences could create a combinatorial code by utilizing an identical molecularmechanism for fine-tuning levels of ciliary receptors.
A suggestion implicit in the authors’ findings is that the GPCR traffickingdefects into the neuronal cilia, especially Mchr1, could underlie the obesity phenotypein the tubby mice. Mchr1, the receptor for melanin-concentrating hormone (MCH),is involved in the regulation of feeding and energy balance [18, 19]. However, Mchr1 knockout mice are lean [18, 19], whereas MCH overexpression results in obesity . Thus, in the simplest model, Mchr1 trafficking defect to the cilia shouldmirror its effect on energy balance and cause leanness, rather than obesity, as evidentin the tubby mouse. Dissecting the downstream effectors of Mchr1 in regulatingenergy balance could address the conflicting effects of Mchr1 trafficking on obesity.The best downstream effector implicated in neuronal satiety pathways is the adenylylcyclase, type 3 (ACIII). Mice deficient in ACIII become obese with age, suggesting thatACIII-mediated cAMP signals are critical in the hypothalamus . In line with this observation, downstream effectors of MCHR1 signalinginclude multiple G proteins including Gi, Go and Gq. Thus, MCHR1 inhibits cAMP production stimulated by forskolin and increasesintracellular Ca2+ levels. However, in metabolically active brain slices, itparadoxically increases extracellular signal-regulated kinase (ERK) phosphorylation tolevels above those observed with forskolin alone . Thus, the synergistic effects of Mchr1 signaling on cAMP, Ca2+,and ERK phosphorylation could be important in determining the final outcome on promotingenergy intake.
Another possibility is the role of additional ciliary GPCRs in neuronal satiety centers,and a combination of trafficking defects of these receptors could result in the finalmaturity-onset obesity phenotype. For example, other neuronal GPCRs such as D1, D2, andD5 dopamine receptors are also expressed in neuronal cilia , and were not examined in this study. Besides, it is important to note thatour catalog of GPCRs expressed in neuronal cilia is mostly incomplete. Thus, althoughthe exact molecular explanation for obesity in the tubby (and BBS) mice is farfrom clear, we still favor the hypothesis that mislocalization of other novel,yet-unidentified GPCRs could provide us with a more complete answer in the future.Nevertheless, the final acid test for dissecting the role of ciliary trafficking ofthese individual receptors on neuronal phenotypes would entail detailed engineering ofknock-in mice, expressing ciliary localization-defective variants into theendogenous genomic loci of these receptors.
Another central question is the means by which ciliary signaling impacts neurons, andthe reason why neurons need this signaling organelle in the first place. Currently, thisis best answered in the case of morphogenetic developmental processes involving Shhsignaling, which impacts neuronal differentiation both during embryogenesis and laterstages. For example, Shh signaling in the cilia is fundamentally important in the neuralprogenitor cells during neural tube patterning . Many of the Shh signaling components are localized to the cilia-basal bodycomplex, and downstream signaling mediated by protein kinase A (PKA) and Gli3 processingare intricately linked to this organelle. In a broader developmental context, primarycilia are also fundamentally important in neurogenesis in cerebellar granule neurons [26, 27], hippocampal neurogenesis in the dentate gyrus (DG) [28, 29], adult DG neural stem cells , and in cerebral cortical development [31, 32]. At least, in some of these neuronal cells, the primary cilium probably actsas a subcellular compartment for efficiently amplifying extracellular Shh signals forintracellular signal transduction. However, neither tubby nor BBS micedemonstrate gross defects in the neuroanatomical networks that regulate satiety,suggesting by extension that a lack of GPCR trafficking in these neurons probably wouldnot cause apparent deficits during development of these circuits.
Apart from the role of cilia in Shh signaling and differentiation, recent studies havebegun to provide intriguing molecular insights into other neuron-dependent processesdependent on the presence of cilia, and similar mechanisms could impact the satietynetworks in a cilia-dependent manner. First of all, primary cilia function inglutamatergic synaptic integration of adult-born neurons . Conditional deletion of cilia from adult-born neurons induces severe defectsin dendritic refinement and synapse formation, which is partially correlated with anenhancement of Wnt and β-catenin signaling . Signaling in the context of primary cilia could thus eventually impinge uponthe subsequent efficient integration of neurons into neural networks. Second, primaryciliary signaling has also been shown to have an effect on long-term potentiation (LTP)and plasticity . Sstr3 signaling in the hippocampus is important in novelty detection inmice, and adenylyl cyclase/cAMP-mediated LTP is impaired in hippocampal slices from theSstr3 knockout and upon addition of Sstr3 antagonists into wild-typesections. In this case, cilia could act as coincidence detectors and affect synapticplasticity by affecting downstream signaling pathways. On a similar note, dopamineproduces a synapse-specific enhancement of early LTP through D1/D5 receptors and cAMPsignaling . Future work is needed to establish if efficient targeting of these receptorsto the neuronal cilia is important in these processes. Identifying downstream pathwaysregulating synaptic plasticity particularly promises to be an important future avenue ofresearch for understanding the puzzling role of cilia in neuronal function.
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