Search

Summary Obesity is a prominent feature of Bardet‐Biedl syndrome (BBS), which represents a major and growing public health problem. More than half of BBS patients carry mutations in one of eight genes that encode subunits of a protein complex known as the BBSome, which has emerged as a key regulator of energy and glucose homeostasis. However, the mechanisms underlying obesity in BBS are complex. Numerous studies have identified a high prevalence of insulin resistance and metabolic syndrome among individuals with BBS. However, the exact mechanisms are not fully understood. This review summarized evidence from experiments using mouse and cell models, focusing on the energy imbalance that leads to obesity in patients with BBS. The studies discussed in this review contribute to understanding the functional role of the BBSome in the obesity associated with BBS, laying the foundation for developing new preventive or therapeutic strategies for obese patients. [ABSTRACT FROM AUTHOR]

The nematode Caenorhabditis elegans is well known for its ability to support forward genetic screens to identify molecules involved in neuronal viability and signaling. The proteins involved in C. elegans dopamine (DA) regulation are highly conserved across evolution, with prior work demonstrating that the model can serve as an efficient platform to identify novel genes involved in disease‐associated processes. To identify novel players in DA signaling, we took advantage of a recently developed library of pre‐sequenced mutant nematodes arising from the million mutation project (MMP) to identify strains that display the DA‐dependent swimming‐induced‐paralysis phenotype (Swip). Our screen identified novel mutations in the dopamine transporter encoding gene dat‐1, whose loss was previously used to identify the Swip phenotype, as well as multiple genes with previously unknown connections to DA signaling. Here, we present our isolation and characterization of one of these genes, bbs‐1, previously linked to the function of primary cilia in worms and higher organisms, including humans, and where loss‐of‐function mutations result in a human disorder known as Bardet–Biedl syndrome. Our studies of C. elegans BBS‐1 protein, as well as other proteins that are known to be assembled into a higher order complex (the BBSome) reveal that functional or structural disruption of this complex leads to exaggerated C. elegans DA signaling to produce Swip via a cell‐autonomous mechanism. We provide evidence that not only does the proper function of cilia in C. elegans DA neurons support normal swimming behavior, but also that bbs‐1 maintains normal levels of DAT‐1 trafficking or function via a RHO‐1 and SWIP‐13/MAPK‐15 dependent pathway where mutants may contribute to Swip independent of altered ciliary function. Together, these studies demonstrate novel contributors to DA neuron function in the worm and demonstrate the utility and efficiency of forward genetic screens using the MMP library. [ABSTRACT FROM AUTHOR]

Bardet‐Biedl syndrome (BBS) is a ciliopathy characterized by retinal degeneration, obesity, renal abnormalities, postaxial polydactyly, and developmental defects. Genes mutated in BBS encode for components and regulators of the BBSome, an octameric complex that controls the trafficking of cargos and receptors within the primary cilium. Although both structure and function of the BBSome have been extensively studied, the impact of ubiquitin signaling on BBSome is largely unknown. We identify the E3 ubiquitin ligase PJA2 as a novel resident of the ciliary compartment and regulator of the BBSome. Upon GPCR‐cAMP stimulation, PJA2 ubiquitylates BBSome subunits. We demonstrate that ubiquitylation of BBS1 at lysine 143 increases the stability of the BBSome and promotes its binding to BBS3, an Arf‐like GTPase protein controlling the targeting of the BBSome to the ciliary membrane. Downregulation of PJA2 or expression of a ubiquitylation‐defective BBS1 mutant (BBS1K143R) affects the trafficking of G‐protein‐coupled receptors (GPCRs) and Shh‐dependent gene transcription. Expression of BBS1K143Rin vivo impairs cilium formation, embryonic development, and photoreceptors' morphogenesis, thus recapitulating the BBS phenotype in the medaka fish model. Synopsis: PJA2 is a novel resident of the ciliary compartment. Upon GPCR‐cAMP stimulation, PJA2 ubiquitylates BBSome subunits and controls the assembly and disassembly of BBSome at cilia. Defects in ubiquitylation may induce Bardet‐Biedl syndrome in medaka fish. E3 ubiquitin ligase PJA2 is a component of the ciliary compartment and interacts with subunits of the BBSome.GPCR‐cAMP stimulation induces PJA2‐mediated ubiquitylation of BBS1 and BBS2 and BBSome complex assembly.Downregulation of PJA2 or expression of a ubiquitylation‐defective BBS1 mutant (BBS1K143R) impairs the trafficking of G‐protein‐coupled receptors (GPCRs) and cilium formation.Expression of BBS1K143R affects cilium formation, embryo development, and photoreceptor morphogenesis in the medaka fish model. [ABSTRACT FROM AUTHOR]

Certain ciliary transmembrane and membrane‐associated signaling proteins export from cilia as intraflagellar transport (IFT) cargoes in a BBSome‐dependent manner. Upon reaching the ciliary tip via anterograde IFT, the BBSome disassembles before being reassembled to form an intact entity for cargo phospholipase D (PLD) coupling. During this BBSome remodeling process, Chlamydomonas Rab‐like 4 GTPase IFT27, by binding its partner IFT25 to form the heterodimeric IFT25/27, is indispensable for BBSome reassembly. Here, we show that IFT27 binds IFT25 in an IFT27 nucleotide‐independent manner. IFT25/27 and the IFT subcomplexes IFT‐A and ‐B are irrelevant for maintaining the stability of one another. GTP‐loading onto IFT27 enhances the IFT25/27 affinity for binding to the IFT‐B subcomplex core IFT‐B1 entity in cytoplasm, while GDP‐bound IFT27 does not prevent IFT25/27 from entering and cycling through cilia by integrating into IFT‐B1. Upon at the ciliary tip, IFT25/27 cycles on and off IFT‐B1 and this process is irrelevant with the nucleotide state of IFT27. During BBSome remodeling at the ciliary tip, IFT25/27 promotes BBSome reassembly independent of IFT27 nucleotide state, making postremodeled BBSomes available for PLD to interact with. Thus, IFT25/27 facilitates BBSome‐dependent PLD export from cilia via controlling availability of intact BBSomes at the ciliary tip, while IFT27 nucleotide state does not participate in this regulatory event. [ABSTRACT FROM AUTHOR]

Primary cilia, microtubule‐based hair‐like structures protruding from most cells, contain membranes enriched in signaling molecules and function as sensory and regulatory organelles critical for development and tissue homeostasis. Intraflagellar transport (IFT), cilia‐specific bidirectional transport, is required for the assembly, maintenance, and function of cilia. BBSome, the coat complex, acts as the adaptor between the IFT complex and membrane proteins and is therefore essential for establishing the specific compartmentalization of signaling molecules in the cilia. Recent findings have revealed that three ciliary Rab‐like small GTPases, IFT27, IFT22, and Rabl2, play critical regulatory roles in ciliary BBSome transport. In this review, we provide an overview of these three Rab‐like small GTPases and their relationship with BBSome. [ABSTRACT FROM AUTHOR]

Adipose tissue is central to the regulation of energy balance. While white adipose tissue (WAT) is responsible for triglyceride storage, brown adipose tissue specializes in energy expenditure. Deterioration of brown adipocyte function contributes to the development of metabolic complications like obesity and diabetes. These disorders are also leading symptoms of the Bardet–Biedl syndrome (BBS), a hereditary disorder in humans which is caused by dysfunctions of the primary cilium and which therefore belongs to the group of ciliopathies. The cilium is a hair‐like organelle involved in cellular signal transduction. The BBSome, a supercomplex of several Bbs gene products, localizes to the basal body of cilia and is thought to be involved in protein sorting to and from the ciliary membrane. The effects of a functional BBSome on energy metabolism and lipid mobilization in brown and white adipocytes were tested in whole‐body Bbs4 knockout mice that were subjected to metabolic challenges. Chronic cold exposure reveals cold‐intolerance of knockout mice but also ameliorates the markers of metabolic pathology detected in knockouts prior to cold. Hepatic triglyceride content is markedly reduced in knockout mice while circulating lipids are elevated, altogether suggesting that defective lipid metabolism in adipose tissue creates increased demand for systemic lipid mobilization to meet energetic demands of reduced body temperatures. These findings taken together suggest that Bbs4 is essential for the regulation of adipose tissue lipid metabolism, representing a potential target to treat metabolic disorders. [ABSTRACT FROM AUTHOR]

Cilia of higher animals sense various environmental stimuli. Proper ciliary signaling requires appropriate extent of BBSome‐mediated export of membrane receptors across ciliary barrier transition zone (TZ) through retrograde intraflagellar transport (IFT) machinery. How the barrier passage is controlled, however, remains unknown. Here, we show that small GTPase Rabl2 functions as a molecular switch for the outward TZ passage. Rabl2‐GTP enters cilia by binding to IFT‐B complex. Its GTP hydrolysis enables the outward TZ passage of the BBSome and its cargos with retrograde IFT machinery, whereas its persistent association leads to their shedding from IFT‐B during the passing process and consequently ciliary retention. Rabl2 deficiency or expression of a GTP‐locked mutant impairs the ciliary hedgehog signaling without interfering with ciliation and respectively results in different spectrums of mouse developmental disorders. We propose that the switch role of Rabl2 ensures proper turnover of the BBSome and ciliary membrane receptors to fine‐tune cilia‐dependent signaling for normal embryonic development and organismic homeostasis. Synopsis: Cilliary signaling is adjusted via retention or export of ciliary receptors through the ciliary transition zone. The small GTPase Rabl2 fine‐tunes this process by GTP hydrolysis‐dependent gating of BBSome‐mediated membrane receptor export. Murine Rabl2 is dispensable for ciliogenesis and the ciliary entry of the IFT‐B ciliary transport complex.Rabl2‐GTP enters cilia by binding to IFT‐B.Ciliary Rabl2‐GTP impairs the exit of the BBSome complex and its cargos across the transition zone.Mice deficient in Rabl2 or expressing an active mutant show distinct developmental defects. [ABSTRACT FROM AUTHOR]

Arl6/BBS3 is a small GTPase, mutations in which are implicated in the human ciliopathy Bardet-Biedl Syndrome (BBS). Arl6 is proposed to facilitate the recruitment of a large protein complex known as the BBSome to the base of the primary cilium, mediating specific trafficking of molecules to this important sensory organelle. Orthologues of Arl6 and the BBSome core subunits have been identified in the genomes of trypanosomes. Flagellum function and motility are crucial to the survival of Trypanosoma brucei, the causative agent of human African sleeping sickness, in the human bloodstream stage of its lifecycle and so the function of the BBSome proteins in trypanosomes warrants further study. RNAi knockdown of T. brucei Arl6 ( TbArl6) has recently been shown to result in shortening of the trypanosome flagellum. Here we present the crystal structure of TbArl6 with the bound non-hydrolysable GTP analog GppNp at 2.0 Å resolution and highlight important differences between the trypanosomal and human proteins. Analysis of the TbArl6 active site confirms that it lacks the key glutamine that activates the nucleophile during GTP hydrolysis in other small GTPases. Furthermore, the trypanosomal proteins are significantly shorter at their N-termini suggesting a different method of membrane insertion compared to humans. Finally, analysis of sequence conservation suggests two surface patches that may be important for protein-protein interactions. Our structural analysis thus provides the basis for future biochemical characterisation of this important family of small GTPases. [ABSTRACT FROM AUTHOR]