Leptin resistance contributes to obesity and hypertension in mouse models of Bardet-Biedl syndrome

Emerging mechanistic understanding of cilia function in cellular signalling

Primary cilia are solitary, immotile sensory organelles present on most cells in the body that participate broadly in human health, physiology and disease. Cilia generate a unique environment for signal transduction with tight control of protein, lipid and second messenger concentrations within a relatively small compartment, enabling reception, transmission and integration of biological information. In this Review, we discuss how cilia function as signalling hubs in cell-cell communication using three signalling pathways as examples: ciliary G-protein-coupled receptors (GPCRs), the Hedgehog (Hh) pathway and polycystin ion channels. We review how defects in these ciliary signalling pathways lead to a heterogeneous group of conditions known as 'ciliopathies', including metabolic syndromes, birth defects and polycystic kidney disease. Emerging understanding of these pathways' transduction mechanisms reveals common themes between these cilia-based signalling pathways that may apply to other pathways as well. These mechanistic insights reveal how cilia orchestrate normal and pathophysiological signalling outputs broadly throughout human biology.

Adipocyte-specific disruption of the BBSome causes metabolic and autonomic dysfunction

Obesity is a major public health issue due to its association with type 2 diabetes, hypertension, and other cardiovascular risks. The BBSome, a complex of eight conserved Bardet-Biedl syndrome (BBS) proteins, has emerged as a key regulator of energy and glucose homeostasis as well as cardiovascular function. However, the importance of adipocyte BBSome in controlling these physiological processes is not clear. Here, we show that adipocyte-specific constitutive disruption of the BBSome through selective deletion of the Bbs1 gene adiponectin (AdipoCre/Bbs1fl/fl mice) does not affect body weight under normal chow or high-fat and high-sucrose diet (HFHSD). However, constitutive BBSome deficiency caused impairment in glucose tolerance and insulin sensitivity. Similar phenotypes were observed after inducible adipocyte-specific disruption of the BBSome (AdipoCreERT2/Bbs1fl/fl mice). Interestingly, a significant increase in renal sympathetic nerve activity, measured using multifiber recording in the conscious state, was observed in AdipoCre/Bbs1fl/fl mice on both chow and HFHSD. A significant increase in tail-cuff arterial pressure was also observed in chow-fed AdipoCre/Bbs1fl/fl mice, but this was not reproduced when arterial pressure was measured by radiotelemetry. Moreover, AdipoCre/Bbs1fl/fl mice had no significant alterations in vascular reactivity. On the other hand, AdipoCre/Bbs1fl/fl mice displayed impaired baroreceptor reflex sensitivity when fed HFHSD, but not on normal chow. Taken together, these data highlight the relevance of the adipocyte BBSome for the regulation of glucose homeostasis and sympathetic traffic. The BBSome also contributes to baroreflex sensitivity under HFHSD, but not normal chow.NEW & NOTEWORTHY The current study show how genetic manipulation of fat cells impacts various functions of the body including sensitivity to the hormone insulin.

A Comprehensive Review of Syndromic Forms of Obesity: Genetic Etiology, Clinical Features and Molecular Diagnosis

Syndromic obesity refers to obesity occurring with additional clinical findings, such as intellectual disability/developmental delay, dysmorphic features, and congenital malformations. PURPOSE OF REVIEW: To present a narrative review regarding the genetic etiology, clinical description, and molecular diagnosis of syndromic obesity, which is a rare condition with high phenotypic variability and genetic heterogeneity. The following syndromes are presented in this review: Prader-Willi, Bardet-Biedl, Pseudohypoparathyroidism, Alström, Smith-Magenis, Cohen, Temple, 1p36 deletion, 16p11.2 microdeletion, Kleefstra, SIM1-related, Börjeson-Forssman-Lehmann, WAGRO, Carpenter, MORM, and MYT1L-related syndromes. RECENT FINDINGS: There are three main groups of mechanisms for syndromic obesity: imprinting, transcriptional activity regulation, and cellular cilia function. For molecular diagnostic, methods of genome-wide investigation should be prioritized over sequencing of panels of syndromic obesity genes. In addition, we present novel syndromic conditions that need further delineation, but evidences suggest they have a higher frequency of obesity. The etiology of syndromic obesity tends to be linked to disrupted neurodevelopment (central) and is associated with a diversity of genes and biological pathways. In the genetic investigation of individuals with syndromic obesity, the possibility that the etiology of the syndromic condition is independent of obesity should be considered. The accurate genetic diagnosis impacts medical management, treatment, and prognosis, and allows proper genetic counseling.

Bardet-Biedl syndrome: A focus on genetics, mechanisms and metabolic dysfunction

Bardet-Biedl syndrome (BBS) is a rare, monogenic, multisystem disorder characterized by retinal dystrophy, renal abnormalities, polydactyly, learning disabilities, as well as metabolic dysfunction, including obesity and an increased risk of type 2 diabetes. It is a primary ciliopathy, and causative mutations in more than 25 different genes have been described. Multiple cellular mechanisms contribute to the development of the metabolic phenotype associated with BBS, including hyperphagia as a consequence of altered hypothalamic appetite signalling as well as alterations in adipocyte biology promoting adipocyte proliferation and adipogenesis. Within this review, we describe in detail the metabolic phenotype associated with BBS and discuss the mechanisms that drive its evolution. In addition, we review current approaches to the metabolic management of patients with BBS, including the use of weight loss medications and bariatric surgery. Finally, we evaluate the potential of targeting hypothalamic appetite signalling to limit hyperphagia and induce clinically significant weight loss.

POMC Neuron BBSome Regulation of Body Weight is Independent of its Ciliary Function

The BBSome, a complex of several Bardet-Biedl syndrome (BBS) proteins including BBS1, has emerged as a critical regulator of energy homeostasis. Although the BBSome is best known for its involvement in cilia trafficking, through a process that involve BBS3, it also regulates the localization of cell membrane receptors underlying metabolic regulation. Here, we show that inducible Bbs1 gene deletion selectively in proopiomelanocortin (POMC) neurons cause a gradual increase in body weight, which was associated with higher fat mass. In contrast, inducible deletion of Bbs3 gene in POMC neurons failed to affect body weight and adiposity. Interestingly, loss of BBS1 in POMC neurons led to glucose intolerance and insulin insensitivity, whereas BBS3 deficiency in these neurons is associated with slight impairment in glucose handling, but normal insulin sensitivity. BBS1 deficiency altered the plasma membrane localization of serotonin 5-HT2C receptor (5-HT2CR) and ciliary trafficking of neuropeptide Y2 receptor (NPY2R).In contrast, BBS3 deficiency, which disrupted the ciliary localization of the BBSome, did not interfere with plasma membrane expression of 5-HT2CR, but reduced the trafficking of NPY2R to cilia. We also show that deficiency in BBS1, but not BBS3, alters mitochondria dynamics and decreased total and phosphorylated levels of dynamin-like protein 1 (DRP1) protein. Importantly, rescuing DRP1 activity restored mitochondria dynamics and localization of 5-HT2CR and NPY2R in BBS1-deficient cells. The contrasting effects on energy and glucose homeostasis evoked by POMC neuron deletion of BBS1 versus BBS3 indicate that BBSome regulation of metabolism is not related to its ciliary function in these neurons.

Metabolic consequences of skeletal muscle- and liver-specific BBSome deficiency

The BBSome is a protein complex composed of eight Bardet-Biedl syndrome (BBS) proteins including BBS1. Humans and mice lacking a functional BBSome display obesity and type 2 diabetes, highlighting the importance of this protein complex for metabolic regulation. However, the contribution of the BBSome in insulin-sensitive tissues such as skeletal muscle and liver to metabolic regulation is ill-defined. Here, we show that disruption of the BBSome through Bbs1 gene deletion in the skeletal muscle had no effect on body weight or glucose handling, but improved insulin sensitivity of female mice without changing insulin receptor signaling. Interestingly, when fed an obesogenic diet, male mice lacking the Bbs1 gene in skeletal muscle exhibited heightened insulin sensitivity despite the comparable weight gain and glucose tolerance relative to controls. On the other hand, normal chow-fed mice missing the Bbs1 gene in hepatocytes displayed increased body weight, as well as impaired glucose handling and insulin sensitivity. This was associated with attenuated insulin signaling in liver and hepatocytes, but not skeletal muscle and white adipose tissue. Moreover, hepatocytes lacking the Bbs1 gene displayed significant reduction in plasma membrane insulin receptor levels due to the mitochondrial dysfunction evoked by loss of the BBSome. Together, these findings demonstrate that myocyte BBSome is minimally involved in metabolic regulation, whereas the hepatic BBSome plays a critical role in the control of energy homeostasis and insulin sensitivity through its requirement for insulin receptor trafficking.NEW & NOTEWORTHY The ongoing epidemic of obesity and associated illnesses highlights the need to understand the biological processes that regulate energy balance. Here, we identified an important role for a protein complex called BBSome in the control of hepatic function. We show that the liver BBSome is necessary to maintain body weight and blood glucose levels due to its requirements to generate energy and detect insulin, a hormone that is essential for metabolic regulation.

Adipose Tissue Hyperplasia and Hypertrophy in Common and Syndromic Obesity-The Case of BBS Obesity

Obesity is a metabolic state generated by the expansion of adipose tissue. Adipose tissue expansion depends on the interplay between hyperplasia and hypertrophy, and is mainly regulated by a complex interaction between genetics and excess energy intake. However, the genetic regulation of adipose tissue expansion is yet to be fully understood. Obesity can be divided into common multifactorial/polygenic obesity and monogenic obesity, non-syndromic and syndromic. Several genes related to obesity were found through studies of monogenic non-syndromic obesity models. However, syndromic obesity, characterized by additional features other than obesity, suggesting a more global role of the mutant genes related to the syndrome and, thus, an additional peripheral influence on the development of obesity, were hardly studied to date in this regard. This review summarizes present knowledge regarding the hyperplasia and hypertrophy of adipocytes in common obesity. Additionally, we highlight the scarce research on syndromic obesity as a model for studying adipocyte hyperplasia and hypertrophy, focusing on Bardet-Biedl syndrome (BBS). BBS obesity involves central and peripheral mechanisms, with molecular and mechanistic alternation in adipocyte hyperplasia and hypertrophy. Thus, we argue that using syndromic obesity models, such as BBS, can further advance our knowledge regarding peripheral adipocyte regulation in obesity.

Modelling renal defects in Bardet-Biedl syndrome patients using human iPS cells

Bardet-Biedl syndrome (BBS) is a ciliopathy with pleiotropic effects on multiple tissues, including the kidney. Here we have compared renal differentiation of iPS cells from healthy and BBS donors. High content image analysis of WT1-expressing kidney progenitors showed that cell proliferation, differentiation and cell shape were similar in healthy, BBS1, BBS2, and BBS10 mutant lines. We then examined three patient lines with BBS10 mutations in a 3D kidney organoid system. The line with the most deleterious mutation, with low BBS10 expression, expressed kidney marker genes but failed to generate 3D organoids. The other two patient lines expressed near normal levels of BBS10 mRNA and generated multiple kidney lineages within organoids when examined at day 20 of organoid differentiation. However, on prolonged culture (day 27) the proximal tubule compartment degenerated. Introducing wild type BBS10 into the most severely affected patient line restored organoid formation, whereas CRISPR-mediated generation of a truncating BBS10 mutation in a healthy line resulted in failure to generate organoids. Our findings provide a basis for further mechanistic studies of the role of BBS10 in the kidney.

An evaluation of setmelanotide injection for chronic weight management in adult and pediatric patients with obesity due to Bardet-Biedl syndrome

INTRODUCTION

Bardet-Biedl Syndrome (BBS) is a rare, multisystemic ciliopathy with an incidence of obesity of 89%. Mutations in genes encoding BBS proteins are linked to reduced leptin sensitivity of hypothalamic POMC neurons and reduced activation of the melanocortin-4 receptor (MC4R) pathway due to deficient α-MSH production by hypothalamic POMC neurons. The MC4R pathway is involved in controlling body weight and energy metabolism, and its disruption is linked to hyperphagia and obesity. Setmelanotide is an MC4R agonist that counteracts deficiencies in the MC4R pathway of individuals with BBS.

AREAS COVERED

Data from clinical trials were reviewed along with information available from setmelanotide's approval for treatment of obesity in people ages ≥6y with a clinical diagnosis of BBS.

EXPERT OPINION

Setmelanotide is available as a daily injectable that can be used for amelioration of obesity in people with Bardet-Biedl syndrome. Its cost is substantial, which may limit its use, but among those who respond, setmelanotide can reduce body mass dramatically and potentially improve comorbid conditions associated with obesity. Setmelanotide treatment has generally tolerable side effects, primarily injection site reactions and nausea/vomiting that generally improve with continued use; almost all people using setmelanotide experience marked skin darkening due to off-target activation of cutaneous MC1R.

Neuronal primary cilia integrate peripheral signals with metabolic drives

Neuronal primary cilia have recently emerged as important contributors to the central regulation of energy homeostasis. As non-motile, microtubule-based organelles, primary cilia serve as signaling antennae for metabolic status. The impairment of ciliary structure or function can produce ciliopathies for which obesity is a hallmark phenotype and global ablation of cilia induces non-syndromic adiposity in mouse models. This organelle is not only a hub for metabolic signaling, but also for catecholamine neuromodulation that shapes neuronal circuitry in response to sensory input. The objective of this review is to highlight current research investigating the mechanisms of primary cilium-regulated metabolic drives for maintaining energy homeostasis.

Adipose tissue function and insulin sensitivity in syndromic obesity of Bardet-Biedl syndrome

BACKGROUND

Bardet-Biedl syndrome (BBS) is a rare autosomal recessive syndromic obesity of childhood onset among many other features. To date, the excess risk of metabolic complications of severe early-onset obesity in BBS remains controversial. In-depth investigation of adipose tissue structure and function with detailed metabolic phenotype has not been investigated yet.

OBJECTIVE

To investigate adipose tissue function in BBS.

DESIGN

A prospective cross-sectional study.

MAIN OUTCOME MEASURE

To determine if there are differences in insulin resistance, metabolic profile, adipose tissue function and gene expression in patients with BBS compared to BMI-matched polygenic obese controls.

METHOD

9 adults with BBS and 10 controls were recruited from the national centre for BBS, Birmingham, UK. An in-depth study of adipose tissue structure and function along with insulin sensitivity was performed using hyperinsulinemic-euglycemic clamp studies, adipose tissue microdialysis, histology and RNA sequencing, and measurement of circulating adipokines and inflammatory biomarkers.

RESULTS

Adipose tissue structure, gene expression and in vivo functional analysis between BBS and polygenic obesity cohorts were similar. Using hyperinsulinemic-euglycemic clamp and surrogate markers of insulin resistance, we found no significant differences in insulin sensitivity between BBS and obese controls. Furthermore, no significant changes were noted in an array of adipokines, cytokines, pro-inflammatory markers and adipose tissue RNA transcriptomic.

CONCLUSION

Although childhood-onset extreme obesity is a feature of BBS, detailed studies of insulin sensitivity and adipose tissue structure and function are similar to common polygenic obesity. This study adds to the literature by suggesting that it is the quality and quantity of adiposity not the duration that drives the metabolic phenotype.

Peripheral and central control of obesity by primary cilia

Primary cilia are hair-like structures that protrude from the cell surface. They are capable of sensing external cues and conveying a vast array of signals into cells to regulate a variety of physiological activities. Mutations in cilium-associated genes are linked to a group of diseases with overlapping clinical manifestations, collectively known as ciliopathies. A significant proportion of human ciliopathy cases are accompanied by metabolic disorders such as obesity and type 2 diabetes. Nevertheless, the mechanisms through which dysfunction of primary cilia contributes to obesity are complex. In this article, we present an overview of primary cilia and highlight obesity-related ciliopathies. We also discuss the potential role of primary cilia in peripheral organs, with a focus on adipose tissues. In addition, we emphasize the significance of primary cilia in the central regulation of obesity, especially the involvement of ciliary signaling in the hypothalamic control of feeding behavior. This article therefore proposes a framework of both peripheral and central regulation of obesity by primary cilia, which may benefit further exploration of the ciliary role in metabolic regulation.

Changes in gut microbiota and short-chain fatty acids are involved in the process of canine obesity after neutering

Neutering is a significant risk factor for obesity in dogs. Changes in gut microbiota and its metabolites have been identified as a key player during obesity progression. However, the mechanisms that promote neuter-associated weight gain are not well understood. Therefore, in this study, sixteen clinically healthy Beagle dogs (6 male and 10 female, mean age = 8.22 ± 0.25 mo old) were neutered. Body weight (BW) and body condition score (BCS) were recorded at 1 d before neutering, 3, 6, 10, 16, and 21 mo after neutering. Dogs were grouped based on their BCS as ideal weight group (IW, n = 4, mean BW = 13.22 ± 1.30 kg, mean BCS = 5.00 ± 0.41) and obese group (OB, n = 12, mean BW = 18.57 ± 1.08 kg, mean BCS = 7.92 ± 0.82) at 21 mo after neutering. Serum lipid profile, glucose, and hormones and fecal microbiota and short-chain fatty acids (SCFAs) were measured. Our results showed that OB dogs had greater (P < 0.0001) BW (18.57 vs. 13.22 kg), BCS (7.92 vs. 5.00), and average daily gain (12.27 vs. 5.69 g/d) than IW dogs at 21 mo after neutering, and the obesity rate was up to 60%. In addition, significant increases (P < 0.05) in serum triglyceride (TG, 1.10 vs. 0.56 mmol/L) and high-density lipoprotein cholesterol (HDL-C, 6.96 vs. 5.40 mmol/L) levels and a significant decrease (P < 0.05) in serum adiponectin (APN, 54.06 vs. 58.39 μg/L) level were observed in OB dogs; serum total cholesterol (4.83 vs. 3.75 mmol/L) (P = 0.075) and leptin (LEP, 2.82 vs. 2.53 μg/L) (P = 0.065) levels tended to be greater in OB dogs; there was a trend towards a lower (P = 0.092) APN/LEP (19.32 vs. 21.81) in OB dogs. Results of fecal microbial alpha-diversity showed that Observed_species and Chao1 indices tended to be lower (P = 0.069) in OB dogs. The STAMP and LEfSe analyses revealed that OB dogs had a greater (P < 0.05 and LDA > 2) reduction in relative abundances of Bacteroides, Prevotella_9, and Megamonas than IW dogs. In addition, OB dogs also had greater (P < 0.05) reduction in fecal acetate, propionate, and butyrate concentrations than IW dogs. Moreover, clear negative correlations (|r| > 0.5 and P < 0.05) were found between SCFAs-producing bacteria and BW, TG, and HDL-C. The functional predictions of microbial communities based on PICRUSt2 analysis revealed that lipid metabolism and endocrine system were significantly disturbed in obese dogs after neutering. Thus, intervention with SCFAs-producing bacteria might represent a new target for the prevention or treatment of canine obesity after neutering. Moreover, weight control before neutering may also contribute to the prevention of canine obesity after neutering.

The BBSome regulates mitochondria dynamics and function

OBJECTIVE

The essential role of mitochondria in regulation of metabolic function and other physiological processes has garnered enormous interest in understanding the mechanisms controlling the function of this organelle. We assessed the role of the BBSome, a protein complex composed of eight Bardet-Biedl syndrome (BBS) proteins, in the control of mitochondria dynamic and function.

METHODS

We used a multidisciplinary approach that include CRISPR/Cas9 technology-mediated generation of a stable Bbs1 gene knockout hypothalamic N39 neuronal cell line. We also analyzed the phenotype of BBSome deficient mice in presence or absence of the gene encoding A-kinase anchoring protein 1 (AKAP1).

RESULTS

Our data show that the BBSome play an important role in the regulation of mitochondria dynamics and function. Disruption of the BBSome cause mitochondria hyperfusion in cell lines, fibroblasts derived from patients as well as in hypothalamic neurons and brown adipocytes of mice. The morphological changes in mitochondria translate into functional abnormalities as indicated by the reduced oxygen consumption rate and altered mitochondrial distribution and calcium handling. Mechanistically, we demonstrate that the BBSome modulates the activity of dynamin-like protein 1 (DRP1), a key regulator of mitochondrial fission, by regulating its phosphorylation and translocation to the mitochondria. Notably, rescuing the decrease in DRP1 activity through deletion of one copy of the gene encoding AKAP1 was effective to normalize the defects in mitochondrial morphology and activity induced by BBSome deficiency. Importantly, this was associated with improvement in several of the phenotypes caused by loss of the BBSome such as the neuroanatomical abnormalities, metabolic alterations and obesity highlighting the importance of mitochondria defects in the pathophysiology of BBS.

CONCLUSIONS

These findings demonstrate a critical role of the BBSome in the modulation of mitochondria function and point to mitochondrial defects as a key disease mechanism in BBS.

Ciliary control of adipocyte progenitor cell fate regulates energy storage

The primary cilium is a cellular sensory organelle found in most cells in our body. This includes adipocyte progenitor cells in our adipose tissue, a complex organ involved in energy storage, endocrine signaling, and thermogenesis. Numerous studies have shown that the primary cilium plays a critical role in directing the cell fate of adipocyte progenitor cells in multiple adipose tissue types. Accordingly, diseases with dysfunctional cilia called ciliopathies have a broad range of clinical manifestations, including obesity and diabetes. This review summarizes our current understanding of how the primary cilium regulates adipocyte progenitor cell fate in multiple contexts and illustrates the importance of the primary cilium in regulating energy storage and adipose tissue function.

Progressive retinal degeneration of rods and cones in a Bardet-Biedl syndrome type 10 mouse model

Bardet-Biedl syndrome (BBS) is a multi-organ autosomal-recessive disorder caused by mutations in at least 22 different genes. A constant feature is early-onset retinal degeneration leading to blindness. Among the most common forms is BBS type 10 (BBS10), which is caused by mutations in a gene encoding a chaperonin-like protein. To aid in developing treatments, we phenotyped a Bbs10 knockout (Bbs10-/-) mouse model. Analysis by optical coherence tomography (OCT), electroretinography (ERG) and a visually guided swim assay (VGSA) revealed a progressive degeneration (from P19 to 8 months of age) of the outer nuclear layer that is visible by OCT and histology. Cone ERG was absent from at least P30, at which time rod ERG was reduced to 74.4% of control levels; at 8 months, rod ERG was 2.3% of that of controls. VGSA demonstrated loss of functional vision at 9 months. These phenotypes progressed more rapidly than retinal degeneration in the Bbs1M390R/M390R knock-in mouse. This study defines endpoints for preclinical trials that can be utilized to detect a treatment effect in the Bbs10-/- mouse and extrapolated to human clinical trials.

Disruption of Dopamine Receptor 1 Localization to Primary Cilia Impairs Signaling in Striatal Neurons

A rod-shaped appendage called a primary cilium projects from the soma of most central neurons in the mammalian brain. The importance of cilia within the nervous system is highlighted by the fact that human syndromes linked to primary cilia dysfunction, collectively termed ciliopathies, are associated with numerous neuropathologies, including hyperphagia-induced obesity, neuropsychiatric disorders, and learning and memory deficits. Neuronal cilia are enriched with signaling molecules, including specific G-protein-coupled receptors (GPCRs) and their downstream effectors, suggesting that they act as sensory organelles that respond to neuromodulators in the extracellular space. We previously showed that GPCR ciliary localization is disrupted in neurons from mouse models of the ciliopathy Bardet-Biedl syndrome (BBS). Based on this finding, we hypothesized that mislocalization of ciliary GPCRs may impact receptor signaling and contribute to the BBS phenotypes. Here, we show that disrupting localization of the ciliary GPCR dopamine receptor 1 (D1) in male and female mice, either by loss of a BBS protein or loss of the cilium itself, specifically in D1-expressing neurons, results in obesity. Interestingly, the weight gain is associated with reduced locomotor activity, rather than increased food intake. Moreover, the loss of a BBS protein or cilia on D1-expressing neurons leads to a reduction in D1-mediated signaling. Together, these results indicate that cilia impact D1 activity in the nervous system and underscore the importance of neuronal cilia for proper GPCR signaling.SIGNIFICANCE STATEMENT Most mammalian neurons possess solitary appendages called primary cilia. These rod-shaped structures are enriched with signaling proteins, such as G-protein-coupled receptors (GPCRs), suggesting that they respond to neuromodulators. This study examines the consequences of disrupting ciliary localization of the GPCR dopamine receptor 1 (D1) in D1-expressing neurons. Remarkably, mice that have either an abnormal accumulation of D1 in cilia or a loss of D1 ciliary localization become obese. In both cases, the obesity is associated with lower locomotor activity rather than overeating. As D1 activation increases locomotor activity, these results are consistent with a reduction in D1 signaling. Indeed, we found that D1-mediated signaling is reduced in brain slices from both mouse models. Thus, cilia impact D1 signaling in the brain.

Palmitic acid control of ciliogenesis modulates insulin signaling in hypothalamic neurons through an autophagy-dependent mechanism

Palmitic acid (PA) is significantly increased in the hypothalamus of mice, when fed chronically with a high-fat diet (HFD). PA impairs insulin signaling in hypothalamic neurons, by a mechanism dependent on autophagy, a process of lysosomal-mediated degradation of cytoplasmic material. In addition, previous work shows a crosstalk between autophagy and the primary cilium (hereafter cilium), an antenna-like structure on the cell surface that acts as a signaling platform for the cell. Ciliopathies, human diseases characterized by cilia dysfunction, manifest, type 2 diabetes, among other features, suggesting a role of the cilium in insulin signaling. Cilium depletion in hypothalamic pro-opiomelanocortin (POMC) neurons triggers obesity and insulin resistance in mice, the same phenotype as mice deficient in autophagy in POMC neurons. Here we investigated the effect of chronic consumption of HFD on cilia; and our results indicate that chronic feeding with HFD reduces the percentage of cilia in hypothalamic POMC neurons. This effect may be due to an increased amount of PA, as treatment with this saturated fatty acid in vitro reduces the percentage of ciliated cells and cilia length in hypothalamic neurons. Importantly, the same effect of cilia depletion was obtained following chemical and genetic inhibition of autophagy, indicating autophagy is required for ciliogenesis. We further demonstrate a role for the cilium in insulin sensitivity, as cilium loss in hypothalamic neuronal cells disrupts insulin signaling and insulin-dependent glucose uptake, an effect that correlates with the ciliary localization of the insulin receptor (IR). Consistently, increased percentage of ciliated hypothalamic neuronal cells promotes insulin signaling, even when cells are exposed to PA. Altogether, our results indicate that, in hypothalamic neurons, impairment of autophagy, either by PA exposure, chemical or genetic manipulation, cause cilia loss that impairs insulin sensitivity.

Cilia Action in Islets: Lessons From Mouse Models

Primary cilia as a signaling organelle have garnered recent attention as a regulator of pancreatic islet function. These rod-like sensors exist on all major islet endocrine cell types and transduce a variety of external cues, while dysregulation of cilia function contributes to the development of diabetes. The complex role of islet primary cilia has been examined using genetic deletion targeting various components of cilia. In this review, we summarize experimental models for the study of islet cilia and current understanding of mechanisms of cilia regulation of islet hormone secretion. Consensus from these studies shows that pancreatic cilia perturbation can cause both endocrine and exocrine defects that are relevant to human disease. We discuss future research directions that would further elucidate cilia action in distinct groups of islet cells, including paracrine and juxtacrine regulation, GPCR signaling, and endocrine-exocrine crosstalk.

Mechanisms of Weight Control by Primary Cilia

A primary cilium, a hair-like protrusion of the plasma membrane, is a pivotal organelle for sensing external environmental signals and transducing intracellular signaling. An interesting linkage between cilia and obesity has been revealed by studies of the human genetic ciliopathies Bardet-Biedl syndrome and Alström syndrome, in which obesity is a principal manifestation. Mouse models of cell type-specific cilia dysgenesis have subsequently demonstrated that ciliary defects restricted to specific hypothalamic neurons are sufficient to induce obesity and hyperphagia. A potential mechanism underlying hypothalamic neuron cilia-related obesity is impaired ciliary localization of G protein-coupled receptors involved in the regulation of appetite and energy metabolism. A well-studied example of this is melanocortin 4 receptor (MC4R), mutations in which are the most common cause of human monogenic obesity. In the paraventricular hypothalamus neurons, a blockade of ciliary trafficking of MC4R as well as its downstream ciliary signaling leads to hyperphagia and weight gain. Another potential mechanism is reduced leptin signaling in hypothalamic neurons with defective cilia. Leptin receptors traffic to the periciliary area upon leptin stimulation. Moreover, defects in cilia formation hamper leptin signaling and actions in both developing and differentiated hypothalamic neurons. The list of obesity-linked ciliary proteins is expending and this supports a tight association between cilia and obesity. This article provides a brief review on the mechanism of how ciliary defects in hypothalamic neurons facilitate obesity.

Mediators of Amylin Action in Metabolic Control

Amylin (also called islet amyloid polypeptide (IAPP)) is a pancreatic beta-cell hormone that is co-secreted with insulin in response to nutrient stimuli. The last 35 years of intensive research have shown that amylin exerts important physiological effects on metabolic control. Most importantly, amylin is a physiological control of meal-ending satiation, and it limits the rate of gastric emptying and reduces the secretion of pancreatic glucagon, in particular in postprandial states. The physiological effects of amylin and its analogs are mediated by direct brain activation, with the caudal hindbrain playing the most prominent role. The clarification of the structure of amylin receptors, consisting of the calcitonin core receptor plus receptor-activity modifying proteins, aided in the development of amylin analogs with a broad pharmacological profile. The general interest in amylin physiology and pharmacology was boosted by the finding that amylin is a sensitizer to the catabolic actions of leptin. Today, amylin derived analogs are considered to be among the most promising approaches for the pharmacotherapy against obesity. At least in conjunction with insulin, amylin analogs are also considered important treatment options in diabetic patients, so that new drugs may soon be added to the only currently approved compound pramlintide (Symlin^®). This review provides a brief summary of the physiology of amylin’s mode of actions and its role in the control of the metabolism, in particular energy intake and glucose metabolism.

BBSome: a New Player in Hypertension and Other Cardiovascular Risks

The BBSome is an octameric protein complex involved in Bardet-Biedl syndrome (BBS), a human pleiotropic, autosomal recessive condition. Patients with BBS display various clinical features including obesity, hypertension, and renal abnormalities. Association studies have also linked the BBS genes to hypertension and other cardiovascular risks in the general population. The BBSome was originally associated with the function of cilia, a highly specialized organelle that extend from the cell membrane of most vertebrate cells. However, subsequent studies have implicated the BBSome in the control of a myriad of other cellular processes not related to cilia including cell membrane localization of receptors and gene expression. The development of animal models of BBS such as mouse lines lacking various components of the BBSome and associated proteins has facilitated studying their role in the control of cardiovascular function and deciphering the pathophysiological mechanisms responsible for the cardiovascular aberrations associated with BBS. These studies revealed the importance of the neuronal, renal, vascular, and cardiac BBSome in the regulation of blood pressure, renal function, vascular reactivity, and cardiac development. The BBSome has also emerged as a critical regulator of key systems involved in cardiovascular control including the renin-angiotensin system. Better understanding of the influence of the BBSome on the molecular and physiological processes relevant to cardiovascular health and disease has the potential of identifying novel mechanisms underlying hypertension and other cardiovascular risks.

Leptin: A Potential Link Between Obstructive Sleep Apnea and Obesity

Chronic intermittent hypoxia (CIH), a pathophysiological manifestation of obstructive sleep apnea (OSA), is strongly correlated with obesity, as patients with the disease experience weight gain while exhibiting elevated plasma levels of leptin. This study was done to determine whether a relationship may exist between CIH and obesity, and body energy balance and leptin signaling during CIH. Sprague-Dawley rats were exposed to 96 days of CIH or normoxic control conditions, and were assessed for measures of body weight, food and water intake, and food conversion efficiency. At the completion of the study leptin sensitivity, locomotor activity, fat pad mass and plasma leptin levels were determined within each group. Additionally, the hypothalamic arcuate nucleus (ARC) was isolated and assessed for changes in the expression of proteins associated with leptin receptor signaling. CIH animals were found to have reduced locomotor activity and food conversion efficiency. Additionally, the CIH group had increased food and water intake over the study period and had a higher body weight compared to normoxic controls at the end of the study. Basal plasma concentrations of leptin were significantly elevated in CIH exposed animals. To test whether a resistance to leptin may have occurred in the CIH animals due to the elevated plasma levels of leptin, an acute exogenous (ip) leptin (0.04 mg/kg carrier-free recombinant rat leptin) injection was administered to the normoxic and CIH exposed animals. Leptin injections into the normoxic controls reduced their food intake, whereas CIH animals did not alter their food intake compared to vehicle injected CIH animals. Within ARC, CIH animals had reduced protein expression of the short form of the obese (leptin) receptor (isoform OBR₁₀₀) and showed a trend toward an elevated protein expression of the long form of obese (leptin) receptor (OBRb). In addition, pro-opiomelanocortin (POMC) protein expression was reduced, but increased expression of the phosphorylated extracellular-signal-regulated kinase 1/2 (pERK1/2) and of the suppressor of cytokine signaling 3 (SOCS3) proteins was observed in the CIH group, with little change in phosphorylated signal transducer and activator of transcription 3 (pSTAT3). Taken together, these data suggest that long-term exposure to CIH, as seen in obstructive sleep apnea, may contribute to a state of leptin resistance promoting an increase in body weight.

Primary Cilia Are Critical Regulators of White Adipose Tissue Expansion

The primary cilium is a microtubule-based cellular protrusion found on most mammalian cell types in diverse tissues. It functions as a cellular antenna to sense and transduce a broad range of signals, including odorants, light, mechanical stimuli, and chemical ligands. This diversity in signals requires cilia to display a context and cell type-specific repertoire of receptors. Recently, primary cilia have emerged as critical regulators of metabolism. The importance of primary cilia in metabolic disease is highlighted by the clinical features of human genetic disorders with dysfunctional ciliary signaling, which include obesity and diabetes. This review summarizes the current literature on the role of primary cilia in metabolic disease, focusing on the importance of primary cilia in directing white adipose tissue expansion during obesity.

A Missense Variant in the Bardet-Biedl Syndrome 2 Gene (BBS2) Leads to a Novel Syndromic Retinal Degeneration in the Shetland Sheepdog

Canine progressive retinal atrophy (PRA) describes a group of hereditary diseases characterized by photoreceptor cell death in the retina, leading to visual impairment. Despite the identification of multiple PRA-causing variants, extensive heterogeneity of PRA is observed across and within dog breeds, with many still genetically unsolved. This study sought to elucidate the causal variant for a distinct form of PRA in the Shetland sheepdog, using a whole-genome sequencing approach. Filtering variants from a single PRA-affected Shetland sheepdog genome compared to 176 genomes of other breeds identified a single nucleotide variant in exon 11 of the Bardet-Biedl syndrome-2 gene (BBS2) (c.1222G>C; p.Ala408Pro). Genotyping 1386 canids of 155 dog breeds, 15 cross breeds and 8 wolves indicated the c.1222G>C variant was only segregated within Shetland sheepdogs. Out of 505 Shetland sheepdogs, seven were homozygous for the variant. Clinical history and photographs for three homozygotes indicated the presence of a novel phenotype. In addition to PRA, additional clinical features in homozygous dogs support the discovery of a novel syndromic PRA in the breed. The development and utilization of a diagnostic DNA test aim to prevent the mutation from becoming more prevalent in the breed.

Loss of the ciliary gene Bbs4 results in defective thermogenesis due to metabolic inefficiency and impaired lipid metabolism

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.

Correction of cilia structure and function alleviates multi-organ pathology in Bardet-Biedl syndrome mice

Bardet–Biedl syndrome (BBS) is a pleiotropic autosomal recessive ciliopathy affecting multiple organs. The development of potential disease-modifying therapy for BBS will require concurrent targeting of multi-systemic manifestations. Here, we show for the first time that monosialodihexosylganglioside accumulates in Bbs2^−/− cilia, indicating impairment of glycosphingolipid (GSL) metabolism in BBS. Consequently, we tested whether BBS pathology in Bbs2^−/− mice can be reversed by targeting the underlying ciliary defect via reduction of GSL metabolism. Inhibition of GSL synthesis with the glucosylceramide synthase inhibitor Genz-667161 decreases the obesity, liver disease, retinal degeneration and olfaction defect in Bbs2^−/− mice. These effects are secondary to preservation of ciliary structure and signaling, and stimulation of cellular differentiation. In conclusion, reduction of GSL metabolism resolves the multi-organ pathology of Bbs2^−/− mice by directly preserving ciliary structure and function towards a normal phenotype. Since this approach does not rely on the correction of the underlying genetic mutation, it might translate successfully as a treatment for other ciliopathies.

From leptin to lasers: the past and present of mouse models of obesity

Introduction: Obesity is a prevalent condition that accounts for significant morbidity and mortality across the globe. Despite substantial effort, most obesity pharmacotherapies have proven unsafe or ineffective. The use of obese mouse models provides unique insight into the hormones and mechanisms that regulate appetite and metabolism. Paramount among these models are the 'obese' and 'diabetic' mice that revealed the powerful satiety hormone leptin, revolutionizing obesity research.Areas Covered: In this article, the authors discuss work on leptin therapy, and the clinical response to leptin in humans. The authors describe the use of modern mouse genetics to study targetable mechanisms for genetic forms of human obesity. Additionally, they describe mouse models of neuromodulation and their utility in unraveling neural circuits that govern appetite and metabolism.Expert opinion: Combining past and present models of obesity is required for the development of safe, effective, and impactful obesity therapy. Current research in obesity can benefit from repositories of genetically engineered mouse models to discover interactions between appetitive systems and circuits. Combining leptin therapy with other satiety signals comprising the gut-brain axis is a promising approach to induce significant enduring weight loss.

Hypothalamic primary cilium: A hub for metabolic homeostasis

Obesity is a global health problem that is associated with adverse consequences such as the development of metabolic disorders, including cardiovascular disease, neurodegenerative disorders, and type 2 diabetes. A major cause of obesity is metabolic imbalance, which results from insufficient physical activity and excess energy intake. Understanding the pathogenesis of obesity, as well as other metabolic disorders, is important in the development of methods for prevention and therapy. The coordination of energy balance takes place in the hypothalamus, a major brain region that maintains body homeostasis. The primary cilium is an organelle that has recently received attention because of its role in controlling energy balance in the hypothalamus. Defects in proteins required for ciliary function and formation, both in humans and in mice, have been shown to cause various metabolic disorders. In this review, we provide an overview of the critical functions of primary cilia, particularly in hypothalamic areas, and briefly summarize the studies on the primary roles of cilia in specific neurons relating to metabolic homeostasis.

BBSome ablation in SF1 neurons causes obesity without comorbidities

OBJECTIVES

The hypothalamic ventromedial nucleus (VMH) plays a major role in metabolic control, but the molecular mechanisms involved remain poorly defined. We analyzed the relevance of the BBSome, a protein complex composed of 8 Bardet-Biedl syndrome (BBS) proteins including BBS1, in VMH steroidogenic factor 1 (SF1) neurons for the control of energy homeostasis and related physiological processes.

METHODS

We generated mice bearing selective BBSome disruption, through Bbs1 gene deletion, in SF1 neurons (SF1Cre/Bbs1fl/fl). We analyzed the consequence on body weight, glucose homeostasis, and cardiovascular autonomic function of BBSome loss in SF1 neurons.

RESULTS

SF1Cre/Bbs1fl/fl mice had increased body weight and adiposity under normal chow conditions. Food intake, energy absorption, and digestive efficiency were not altered by Bbs1 gene deletion in SF1 neurons. SF1Cre/Bbs1fl/fl mice exhibited lower energy expenditure, particularly during the dark cycle. Consistent with this finding, SF1Cre/Bbs1fl/fl mice displayed reduced sympathetic nerve traffic and expression of markers of thermogenesis in brown adipose tissue. SF1Cre/Bbs1fl/fl mice also had lower sympathetic nerve activity to subcutaneous white adipose tissue that was associated with a protein expression profile that promotes lipid accumulation. Notably, despite obesity and hyperinsulinemia, SF1Cre/Bbs1fl/fl mice did not exhibit significant changes in glucose metabolism, insulin sensitivity, blood pressure, and baroreflex sensitivity.

CONCLUSIONS

Our findings demonstrate that the SF1 neuron BBSome is necessary for the regulation of energy homeostasis through modulation of the activity of the sympathetic nervous system and that the SF1 neuron BBSome is required for the development of obesity-related comorbidities.

Melanocortin 4 receptor signals at the neuronal primary cilium to control food intake and body weight

The melanocortin 4 receptor (MC4R) plays a critical role in the long-term regulation of energy homeostasis, and mutations in the MC4R are the most common cause of monogenic obesity. However, the precise molecular and cellular mechanisms underlying the maintenance of energy balance within MC4R-expressing neurons are unknown. We recently reported that the MC4R localizes to the primary cilium, a cellular organelle that allows for partitioning of incoming cellular signals, raising the question of whether the MC4R functions in this organelle. Here, using mouse genetic approaches, we found that cilia were required specifically on MC4R-expressing neurons for the control of energy homeostasis. Moreover, these cilia were critical for pharmacological activators of the MC4R to exert an anorexigenic effect. The MC4R is expressed in multiple brain regions. Using targeted deletion of primary cilia, we found that cilia in the paraventricular nucleus of the hypothalamus (PVN) were essential to restrict food intake. MC4R activation increased adenylyl cyclase (AC) activity. As with the removal of cilia, inhibition of AC activity in the cilia of MC4R-expressing neurons of the PVN caused hyperphagia and obesity. Thus, the MC4R signaled via PVN neuron cilia to control food intake and body weight. We propose that defects in ciliary localization of the MC4R cause obesity in human inherited obesity syndromes and ciliopathies.

Bardet-Biedl syndrome proteins regulate intracellular signaling and neuronal function in patient-specific iPSC-derived neurons

Bardet-Biedl syndrome (BBS) is a rare autosomal recessive disorder caused by mutations in genes encoding components of the primary cilium and is characterized by hyperphagic obesity. To investigate the molecular basis of obesity in human BBS, we developed a cellular model of BBS using induced pluripotent stem cell-derived (iPSC-derived) hypothalamic arcuate-like neurons. BBS mutations BBS1M390R and BBS10C91fsX95 did not affect neuronal differentiation efficiency but caused morphological defects, including impaired neurite outgrowth and longer primary cilia. Single-cell RNA sequencing of BBS1M390R hypothalamic neurons identified several downregulated pathways, including insulin and cAMP signaling and axon guidance. Additional studies demonstrated that BBS1M390R and BBS10C91fsX95 mutations impaired insulin signaling in both human fibroblasts and iPSC-derived neurons. Overexpression of intact BBS10 fully restored insulin signaling by restoring insulin receptor tyrosine phosphorylation in BBS10C91fsX95 neurons. Moreover, mutations in BBS1 and BBS10 impaired leptin-mediated p-STAT3 activation in iPSC-derived hypothalamic neurons. Correction of the BBS mutation by CRISPR rescued leptin signaling. POMC expression and neuropeptide production were decreased in BBS1M390R and BBS10C91fsX95 iPSC-derived hypothalamic neurons. In the aggregate, these data provide insights into the anatomic and functional mechanisms by which components of the BBSome in CNS primary cilia mediate effects on energy homeostasis.

The BBSome: a nexus controlling energy metabolism in the brain

Bardet-Biedl syndrome (BBS) is a syndromic ciliopathy that has obesity as a cardinal feature. BBS is caused by mutations in BBS genes. BBS proteins control primary cilia function, and BBS mutations therefore lead to dysfunctional primary cilia. Obesity in patients with BBS is mainly caused by hyperphagia due to dysregulated neuronal function in the brain, in particular in the hypothalamus. However, the mechanism by which mutations in BBS genes result in dysfunction in hypothalamic neurons is not well understood. In this issue of the JCI, Wang et al. used BBS and non-BBS patient-derived induced pluripotent stem cells to generate neurons and hypothalamic neurons. Using this human model system, the authors demonstrated that mutations in BBS genes affected primary cilia function, neuronal morphology, and signaling pathways regulating the function of hypothalamic neurons, which control energy homeostasis. This study provides important insights into the mechanisms of BBS-induced obesity.

Photoreceptor cilia, in contrast to primary cilia, grant entry to a partially assembled BBSome

The BBSome is a protein complex consisting of BBS1, BBS2, BBS4, BBS5, BBS7, BBS8, BBS9 and BBS18 that associates with intraflagellar transport complexes and specializes in ciliary trafficking. In primary cilia, ciliary entry requires the fully assembled BBSome as well as the small GTPase, ARL6 (BBS3). Retinal photoreceptors possess specialized cilia. In light of key structural and functional differences between primary and specialized cilia, we examined the principles of BBSome recruitment to photoreceptor cilia. We performed sucrose gradient fractionation using retinal lysates of Bbs2-/-, Bbs7-/-, Bbs8-/- and Bbs3-/- mice to determine the status of BBSome assembly, then determined localization of BBSome components using immunohistochemistry. Surprisingly, we found that a subcomplex of the BBSome containing at least BBS1, BBS5, BBS8 and BBS9 is recruited to cilia in the absence of BBS2 or BBS7. In contrast, a BBSome subcomplex consisting of BBS1, BBS2, BBS5, BBS7 and BBS9 is found in Bbs8-/- retinas and is denied ciliary entry in photoreceptor cells. In addition, the BBSome remains fully assembled in Bbs3-/- retinas and can be recruited to photoreceptor cilia in the absence of BBS3. We compared phenotypic severity of their retinal degeneration phenotypes. These findings demonstrate that unlike primary cilia, photoreceptor cilia admit a partially assembled BBSome meeting specific requirements. In addition, the recruitment of the BBSome to photoreceptor cilia does not require BBS3. These findings indicate that the ciliary entry of the BBSome is subjected to cell-specific regulation, particularly in cells with highly adapted forms of cilia such as photoreceptors.

Bardet-Biedl Syndrome ciliopathy is linked to altered hematopoiesis and dysregulated self-tolerance

Bardet-Biedl Syndrome (BBS) is a pleiotropic genetic disease caused by the dysfunction of primary cilia. The immune system of patients with ciliopathies has not been investigated. However, there are multiple indications that the impairment of the processes typically associated with cilia may have influence on the hematopoietic compartment and immunity. In this study, we analyze clinical data of BBS patients and corresponding mouse models carrying mutations in Bbs4 or Bbs18. We find that BBS patients have a higher prevalence of certain autoimmune diseases. Both BBS patients and animal models have altered red blood cell and platelet compartments, as well as elevated white blood cell levels. Some of the hematopoietic system alterations are associated with BBS-induced obesity. Moreover, we observe that the development and homeostasis of B cells in mice is regulated by the transport complex BBSome, whose dysfunction is a common cause of BBS. The BBSome limits canonical WNT signaling and increases CXCL12 levels in bone marrow stromal cells. Taken together, our study reveals a connection between a ciliopathy and dysregulated immune and hematopoietic systems.

Properties of primary cilia in melanin-concentrating hormone receptor 1-bearing hippocampal neurons in vivo and in vitro

The primary cilium is a solitary organelle that organizes a sensitive signaling hub in a highly ordered microenvironment. Cilia are plastic structures, changing their length in response to bioactive substances, and ciliary length may be regulated to ensure efficient signaling capacity. Mammalian brain neurons possess primary cilia that are enriched in a set of G protein-coupled receptors (GPCRs), including the feeding-related melanin-concentrating hormone (MCH) receptor 1 (MCHR1). We previously demonstrated a novel biological phenomenon, ciliary MCHR1-mediated cilia length shortening through Gi/o and Akt signaling, using a simple cell culture model of human retinal pigmented epithelial RPE1 cells exogenously expressing MCHR1. In the present study, we characterized the properties of endogenous MCHR1-expressing primary cilia in hippocampal neurons in rodents. Using cultured dissociated rat hippocampal neurons in vitro, we showed that MCH triggered cilia length reduction involved in MCHR1-Gi/o and -Akt signaling. In rat hippocampal slice cultures with preservation of the cytoarchitecture and cell populations, ciliary MCHR1 was abundantly located in the CA1 and CA3 regions, but not in the dentate gyrus. Notably, treatment of slice cultures with MCH induced Gi/o- and Akt-dependent cilia shortening in the CA1 region without influencing cilia length in the CA3 region. Regarding the in vivo mouse brain, we observed higher levels of ciliary MCHR1 in the CA1 and CA3 regions as well as in slice cultures. In the starved state mice, a marked increase in MCH mRNA expression was detected in the lateral hypothalamus. Furthermore, MCHR1-positive cilia length in the hippocampal CA1 region was significantly shortened in fasted mice compared with fed mice. The present findings focused on the hippocampus provide a potential approach to investigate how MCHR1-driven cilia shortening regulates neuronal activity and physiological function toward feeding and memory tasks.

Potential Roles of O-GlcNAcylation in Primary Cilia- Mediated Energy Metabolism

The primary cilium, an antenna-like structure on most eukaryotic cells, functions in transducing extracellular signals into intracellular responses via the receptors and ion channels distributed along it membrane. Dysfunction of this organelle causes an array of human diseases, known as ciliopathies, that often feature obesity and diabetes; this indicates the primary cilia's active role in energy metabolism, which it controls mainly through hypothalamic neurons, preadipocytes, and pancreatic β-cells. The nutrient sensor, O-GlcNAc, is widely involved in the regulation of energy homeostasis. Not only does O-GlcNAc regulate ciliary length, but it also modifies many components of cilia-mediated metabolic signaling pathways. Therefore, it is likely that O-GlcNAcylation (OGN) plays an important role in regulating energy homeostasis in primary cilia. Abnormal OGN, as seen in cases of obesity and diabetes, may play an important role in primary cilia dysfunction mediated by these pathologies.

There and Back Again: Leptin Actions in White Adipose Tissue

Leptin is a hormone discovered almost 30 years ago with important implications in metabolism. It is primarily produced by white adipose tissue (WAT) in proportion to the amount of fat. The discovery of leptin was a turning point for two principle reasons: on one hand, it generated promising expectations for the treatment of the obesity, and on the other, it changed the classical concept that white adipose tissue was simply an inert storage organ. Thus, adipocytes in WAT produce the majority of leptin and, although its primary role is the regulation of fat stores by controlling lipolysis and lipogenesis, this hormone also has implications in other physiological processes within WAT, such as apoptosis, browning and inflammation. Although a massive number of questions related to leptin actions have been answered, the necessity for further clarification facilitates constantly renewing interest in this hormone and its pathways. In this review, leptin actions in white adipose tissue will be summarized in the context of obesity.

Systemic and Central Amylin, Amylin Receptor Signaling, and Their Physiological and Pathophysiological Roles in Metabolism

This article in the Neural and Endocrine Section of Comprehensive Physiology discusses the physiology and pathophysiology of the pancreatic hormone amylin. Shortly after its discovery in 1986, amylin has been shown to reduce food intake as a satiation signal to limit meal size. Amylin also affects food reward, sensitizes the brain to the catabolic actions of leptin, and may also play a prominent role in the development of certain brain areas that are involved in metabolic control. Amylin may act at different sites in the brain in addition to the area postrema (AP) in the caudal hindbrain. In particular, the sensitizing effect of amylin on leptin action may depend on a direct interaction in the hypothalamus. The concept of central pathways mediating amylin action became more complex after the discovery that amylin is also synthesized in certain hypothalamic areas but the interaction between central and peripheral amylin signaling remains currently unexplored. Amylin may also play a dominant pathophysiological role that is associated with the aggregation of monomeric amylin into larger, cytotoxic molecular entities. This aggregation in certain species may contribute to the development of type 2 diabetes mellitus but also cardiovascular disease. Amylin receptor pharmacology is complex because several distinct amylin receptor subtypes have been described, because other neuropeptides [e.g., calcitonin gene-related peptide (CGRP)] can also bind to amylin receptors, and because some components of the functional amylin receptor are also used for other G-protein coupled receptor (GPCR) systems. © 2020 American Physiological Society. Compr Physiol 10:811-837, 2020.

Bardet-Biedl syndrome and related disorders in Japan

Bardet-Biedl syndrome (BBS) is a rare autosomal recessive disorder characterized by obesity, mental impairment, rod-cone dystrophy, polydactyly, male hypogonadism, and renal abnormalities. This disorder is caused by mutations in BBS1-21. Alström syndrome (AS), caused solely by mutations in ALMS1, is another genetic obesity syndrome clinically similar to BBS. We previously conducted the first nationwide survey of BBS in Japan and found four patients with genetically definite BBS. In this study, exome analyses were performed on new patients whose symptoms fulfilled the diagnostic criteria for BBS. We identified one reported heterozygous mutation in BBS1 (p.R429*) in one patient, two novel mutations (p.L493R and p.H719Y) in BBS20 in a second patient, and one novel mutation (p.Q920*) and one reported mutation (p.R2928*) in ALMS1 in a third patient, who was subsequently diagnosed with AS. The first patient with BBS was previously considered to have digenic heterozygous mutations in BBS1 and BBS4. RT-PCR and long-range genomic PCR analyses identified a new heterozygous mutation in BBS1, the deletion of exons 10 and 11. Thus, this patient was compound heterozygous for mutations in BBS1. Many studies have described digenic heterozygous mutations in BBS. However, undetected mutations might have existed in either one of the mutated genes.

Cilia signaling and obesity

An emerging number of rare genetic disorders termed ciliopathies are associated with pediatric obesity. It is becoming clear that the mechanisms associated with cilia dysfunction and obesity in these syndromes are complex. In addition to ciliopathic syndromic forms of obesity, several cilia-associated signaling gene mutations also lead to morbid obesity. While cilia have critical and diverse functions in energy homeostasis including their roles in centrally mediated food intake as well as in peripheral tissues, many questions remain. Here, we briefly discuss the syndromic ciliopathies and monoallelic cilia signaling gene mutations associated with obesity. We also describe potential ways cilia may be involved in common obesity. We discuss how neuronal cilia impact food intake potentially through leptin signaling and changes in ciliary G protein-coupled receptor (GPCR) signaling. We highlight several recent studies that have implicated the potential for cilia in peripheral tissues such as adipose and the pancreas to contribute to metabolic dysfunction. Then we discuss the potential for cilia to impact energy homeostasis through their roles in both development and adult tissue homeostasis. The studies discussed in this review highlight how a comprehensive understanding of the requirement of cilia for the regulation of diverse biological functions will contribute to our understanding of common forms of obesity.

Cardiovascular Regulation by the Neuronal BBSome

The BBSome, a complex of 8 BBS (Bardet-Biedl syndrome) proteins known for its role in the control of cilia function and other cellular processes, has been implicated in blood pressure control, but the underlying mechanisms are not fully understood. Here, we show that neuronal BBSome plays an important role in blood pressure regulation. Targeted inactivation of the BBSome in the nervous system through Bbs1 gene deletion causes sympathetically mediated increase in blood pressure in mice. This phenotype is reproduced by selective ablation of the Bbs1 gene from the LRb (leptin receptor)-expressing neurons. Strikingly, the well-known role of the BBSome in the regulation of cilia formation and function is unlikely to account for the prohypertensive effect of BBSome inactivation as disruption of the IFT (intraflagellar transport) machinery required for ciliogenesis by deleting the Ift88 gene in LRb neurons had no effect on arterial pressure and sympathetic nerve activity. Furthermore, we found that Bbs1 gene deletion from AgRP (agouti-related protein) neurons or POMC (proopiomelanocortin) neurons increased renal and splanchnic sympathetic nerve activity without altering blood pressure. This lack of blood pressure increase despite the sympathetic overdrive may be explained by vascular adrenergic desensitization as indicated by the reduced vascular contractile response evoked by phenylephrine and the decreased expression of adrenergic receptors. Our results identify the neuronal BBSome as a new player in hemodynamic, sympathetic, and vascular regulation, in a manner independent of cilia.

Bardet-Biedl Syndrome Caused by Skipping of SCLT1 Complicated by Microvesicular Steatohepatitis

We treated the case of a 22-year-old male patient with liver dysfunction. At 1 year of age, hepatic fibrosis was suspected. In addition, due to the presence of retinitis pigmentosa, renal failure, obesity, mental retardation, and hypogonadism, he was diagnosed with Bardet-Biedl syndrome (BBS). Skipping of exons 14 and 17 in the sodium channel and clathrin linker 1 (SCLT1) gene was observed. At 22 years of age, the liver enzyme levels were further elevated and a diagnosis of microvesicular steatohepatitis was made. Insulin resistance, a reduction of muscle mass, an impairment of the fatty acid metabolism, and hyperleptinemia in this syndrome may cause steatohepatitis.

Lztfl1/BBS17 controls energy homeostasis by regulating the leptin signaling in the hypothalamic neurons

Leptin receptor (LepRb) signaling pathway in the hypothalamus of the forebrain controls food intake and energy expenditure in response to an altered energy state. Defects in the LepRb signaling pathway can result in leptin-resistance and obesity. Leucine zipper transcription factor like 1 (Lztfl1)/BBS17 is a member of the Bardet-Biedl syndrome (BBS) gene family. Human BBS patients have a wide range of pathologies including obesity. The cellular and molecular mechanisms underlying Lztfl1-regulated obesity are unknown. Here, we generated Lztfl1f/f mouse model in which Lztfl1 can be deleted globally and in tissue-specific manner. Global Lztfl1 deficiency resulted in pleiotropic phenotypes including obesity. Lztfl1-/- mice are hyperphagic and showed similar energy expenditure as WT littermates. The obese phenotype of Lztfl1-/- mice is caused by the loss of Lztfl1 in the brain but not in the adipocytes. Lztfl1-/- mice are leptin-resistant. Inactivation of Lztfl1 abolished phosphorylation of Stat3 in the LepRb signaling pathway in the hypothalamus upon leptin stimulation. Deletion of Lztfl1 had no effect on LepRb membrane localization. Furthermore, we observed that Lztfl1-/- mouse embryonic fibroblasts (MEFs) have significantly longer cilia than WT MEFs. We identified several proteins that potentially interact with Lztfl1. As these proteins are known to be involved in regulation of actin/cytoskeleton dynamics, we suggest that Lztfl1 may regulate leptin signaling and ciliary structure via these proteins. Our study identified Lztfl1 as a novel player in the LepRb signaling pathway in the hypothalamus that controls energy homeostasis.

The brain-adipocyte-gut network: Linking obesity and depression subtypes

Major depressive disorder (MDD) and obesity are dominant and inter-related health burdens. Obesity is a risk factor for MDD, and there is evidence MDD increases risk of obesity. However, description of a bidirectional relationship between obesity and MDD is misleading, as closer examination reveals distinct unidirectional relationships in MDD subtypes. MDD is frequently associated with weight loss, although obesity promotes MDD. In contrast, MDD with atypical features (MDD-AF) is characterised by subsequent weight gain and obesity. The bases of these distinct associations remain to be detailed, with conflicting findings clouding interpretation. These associations can be viewed within a systems biology framework-the psycho-immune neuroendocrine (PINE) network shared between MDD and metabolic disorders. Shared PINE subsystem perturbations may underlie increased MDD in overweight and obese people (obesity-associated depression), while obesity in MDD-AF (depression-associated obesity) involves more complex interactions between behavioural and biomolecular changes. In the former, the chronic PINE dysfunction triggering MDD is augmented by obesity-dependent dysregulation in shared networks, including inflammatory, leptin-ghrelin, neuroendocrine, and gut microbiome systems, influenced by chronic image-associated psychological stress (particularly in younger or female patients). In MDD-AF, behavioural dysregulation, including hypersensitivity to interpersonal rejection, fundamentally underpins energy imbalance (involving hyperphagia, lethargy, hypersomnia), with evolving obesity exaggerating these drivers via positive feedback (and potentially augmenting PINE disruption). In both settings, sex and age are important determinants of outcome, associated with differences in emotional versus cognitive dysregulation. A systems biology approach is recommended for further research into the pathophysiological networks underlying MDD and linking depression and obesity.

Smooth Muscle Cell-Specific Disruption of the BBSome Causes Vascular Dysfunction

The BBSome-a complex consisting of 8 Bardet-Biedl syndrome proteins-is involved in the regulation of various cellular processes. Recently, the BBSome complex has emerged as an important regulator of cardiovascular function with implications for disease. In this study, we examined the role of the BBSome in vascular smooth muscle and its effects on the regulation of cardiovascular function. Smooth muscle-specific disruption of the BBSome through tamoxifen-inducible deletion of Bbs1 gene-a critical component of the BBSome complex-reduces relaxation and enhances contractility of vascular rings and increases aortic stiffness independent of changes in arterial blood pressure. Mechanistically, we demonstrate that smooth muscle Bbs1 gene deletion increases vascular angiotensinogen gene expression implicating the renin-angiotensin system in these altered cardiovascular responses. Additionally, we report that smooth muscle-specific Bbs1 knockout mice demonstrate enhanced ET-1 (endothelin-1)-induced contractility of mesenteric arteries-an effect reversed by blockade of the AT1 (angiotensin type 1 receptor) with losartan. These findings highlight the importance of the smooth muscle BBSome in the control of vascular function and arterial stiffness through modulation of renin-angiotensin system signaling.

The BBSome in POMC and AgRP Neurons Is Necessary for Body Weight Regulation and Sorting of Metabolic Receptors

The BBSome, a complex of eight Bardet-Biedl syndrome (BBS) proteins involved in cilia function, has emerged as an important regulator of energy balance, but the underlying cellular and molecular mechanisms are not fully understood. Here, we show that the control of energy homeostasis by the anorexigenic proopiomelanocortin (POMC) neurons and orexigenic agouti-related peptide (AgRP) neurons require intact BBSome. Targeted disruption of the BBSome by Bbs1 gene deletion in POMC or AgRP neurons increases body weight and adiposity. We demonstrate that obesity in mice lacking the Bbs1 gene in POMC neurons is associated with hyperphagia. Mechanistically, we present evidence implicating the BBSome in the trafficking of G protein-coupled neuropeptide Y Y2 receptor (NPY₂R) and serotonin 5-hydroxytryptamine (HT)_2C receptor (5-HT_2CR) to cilia and plasma membrane, respectively. Consistent with this, loss of the BBSome reduced cell surface expression of the 5-HT_2CR, interfered with serotonin-evoked increase in intracellular calcium and membrane potential, and blunted the anorectic and weight-reducing responses evoked by the 5-HT_2cR agonist, lorcaserin. Finally, we show that disruption of the BBSome causes the 5-HT_2CR to be stalled in the late endosome. Our results demonstrate the significance of the hypothalamic BBSome for the control of energy balance through regulation of trafficking of important metabolic receptors.

Meta-analysis of genotype-phenotype associations in Bardet-Biedl syndrome uncovers differences among causative genes

Bardet-Biedl syndrome (BBS) is a recessive genetic disease causing multiple organ anomalies. Most patients carry mutations in genes encoding for the subunits of the BBSome, an octameric ciliary transport complex, or accessory proteins involved in the BBSome assembly or function. BBS proteins have been extensively studied using in vitro, cellular, and animal models. However, the molecular functions of particular BBS proteins and the etiology of the BBS symptoms are still largely elusive. In this study, we applied a meta-analysis approach to study the genotype-phenotype association in humans using our database of all reported BBS patients. The analysis revealed that the identity of the causative gene and the character of the mutation partially predict the clinical outcome of the disease. Besides their potential use for clinical prognosis, our analysis revealed functional differences of particular BBS genes in humans. Core BBSome subunits BBS2, BBS7, and BBS9 manifest as more critical for the function and development of kidneys than peripheral subunits BBS1, BBS4, and BBS8/TTC8, suggesting that incomplete BBSome retains residual function at least in the kidney.

Bardet-Biedl syndrome obesity: BBS4 regulates cellular ER stress in early adipogenesis

BACKGROUND

Bardet-Biedl syndrome (BBS) is an autosomal recessive ciliopathy, presenting with early obesity onset. The etiology of BBS obesity involves both central and peripheral defects, through mechanisms mostly yet to be deciphered. We previously showed BBS4 expression in adipogenesis, peaking at day 3 of differentiation. Obesity is characterized by cellular stress which promotes pathological consequences.

AIMS

We set out to test a possible role of BBS4 in adipocyte endoplasmic reticulum (ER) stress-induced unfolding protein response (UPR).

METHODS

BBS4 silenced (SiBBS4) and overexpressing (OEBBS4) pre-adipocyte murine cell lines were subjected to ER-stress induction (Tunicamycin, TM) during adipogenesis. ER-stress UPR was analyzed at the transcript, protein and biochemical levels (microscopy, immunocytochemistry, western blotting, quantitative RT-PCR and X-box binding protein 1 (XBP-1) splicing).

RESULTS

In silico analysis showed that BBS4 harbors an ER localization sequences indicative of ER localization. We verified BBS4's ER localization in adipocytes by immunocytochemistry and cellular protein fractionation. Furthermore, we demonstrated that BBS4 expression is significantly up-regulated by ER-stress, as indicated by protein and transcript levels. SiBBS4 adipocytes exhibited swollen ER typical to ER-stress and significant XBP-1 down-regulation at day 3 of differentiation. Following ER-stress, SiBBS4 adipocytes exhibited XBP-1 ER retention, failure to translocate to the nucleus and depletion of the nuclear active cleaved ATF6α. BBS4 did not alter ATF6α processing by S1P and S2P in the Golgi. Notably, SiBBS4 cells demonstrated significant reduction in the downstream activated phospho-IRE1α, independent of ER-stress.

CONCLUSIONS

At day 3 of adipogenesis, coinciding with the timing of its peak expression, BBS4 is localized to the ER and is involved in the ER stress response and trafficking. BBS4 depletion results in swollen ER with impaired intracellular nucleus translocation of XBP-1 and ATF6α. Thus, BBS4 affects the ER stress response in early adipogenesis, altering ER stress responsiveness and the adipocyte ER phenotype.