A Splice Variant of Bardet-Biedl Syndrome 5 (BBS5) Protein that Is Selectively Expressed in Retina

Changes in expression of mesothelial BBS genes in 2D and 3D after lithium chloride and ammonium sulphate induction of primary cilium disturbance: a pilot study

BACKGROUND

Malignant pleural mesothelioma (MPM), a rare and aggressive pleural tumor, has significant histological and molecular heterogeneity. Primary Cilium (PC), an organelle of emerging importance in malignancies, has been scarcely investigated in MPM. A critical molecular complex for the PC function is the BBSome and here we aimed at assessing its expression patterns in ordinary 2D and spheroid 3D cell cultures.

METHODS

A human benign mesothelial cell line (MeT-5A), MPM cell lines (M14K, epithelioid MPM; MSTO, biphasic MPM), and primary MPM cells (pMPM) were used. Primers specific for the human BBS1, 2, 4, 5, 7, 9, 18 transcripts were designed, and quantitative real-time PCR (qRT-PCR) was done with β-actin as the gene of reference. The relative gene expression across 2D and 3D cultures was analyzed by the expression factor (mean of 1/ΔCt values). With the 2-∆∆Ct method the gene expression fold changes were assessed from qRT-PCR data. Molecular changes using the PC-modulating drugs ammonium sulfate (AS) and lithium chloride (LC) were also determined.

RESULTS

PC was present in all cells used in the study at approximately 15% of the observed area. BBSome transcripts were differentially expressed in different dimensions of cell culture (2D vs. 3D) in all cell lines and pMPM. Treatment with AS and LC affected the expression of the ciliary BBS2 and BBS18 genes in the benign as well as in the MPM cells.

CONCLUSIONS

These data indicate distinct BBSome molecular profiles in human benign and MPM cells cultured in 2D and 3D dimensions and support the notion that PC genes should be investigated as potential MPM therapeutic targets.

The crucial role of model systems in understanding the complexity of cell signaling in human neurocristopathies

Animal models are useful to study the molecular, cellular, and morphogenetic mechanisms underlying normal and pathological development. Cell-based study models have emerged as an alternative approach to study many aspects of human embryonic development and disease. The neural crest (NC) is a transient, multipotent, and migratory embryonic cell population that generates a diverse group of cell types that arises during vertebrate development. The abnormal formation or development of the NC results in neurocristopathies (NCPs), which are characterized by a broad spectrum of functional and morphological alterations. The impaired molecular mechanisms that give rise to these multiphenotypic diseases are not entirely clear yet. This fact, added to the high incidence of these disorders in the newborn population, has led to the development of systematic approaches for their understanding. In this article, we have systematically reviewed the ways in which experimentation with different animal and cell model systems has improved our knowledge of NCPs, and how these advances might contribute to the development of better diagnostic and therapeutic tools for the treatment of these pathologies. This article is categorized under: Congenital Diseases > Genetics/Genomics/Epigenetics Congenital Diseases > Stem Cells and Development Congenital Diseases > Molecular and Cellular Physiology Neurological Diseases > Genetics/Genomics/Epigenetics.

Retinal ciliopathies through the lens of Bardet-Biedl Syndrome: Past, present and future

The primary cilium is a highly specialized and evolutionary conserved organelle in eukaryotes that plays a significant role in cell signaling and trafficking. Over the past few decades tremendous progress has been made in understanding the physiology of cilia and the underlying pathomechanisms of various ciliopathies. Syndromic ciliopathies consist of a group of disorders caused by ciliary dysfunction or abnormal ciliogenesis. These disorders have multiorgan involvement in addition to retinal degeneration underscoring the ubiquitous distribution of primary cilia in different cell types. Genotype-phenotype correlation is often challenging due to the allelic heterogeneity and pleiotropy of these disorders. In this review, we discuss the clinical and genetic features of syndromic ciliopathies with a focus on Bardet-Biedl syndrome (BBS) as a representative disorder. We discuss the structure and function of primary cilia and their role in retinal photoreceptors. We describe the progress made thus far in understanding the functional and genetic characterization including expression quantitative trait locus (eQTL) analysis of BBS genes. In the future directions section, we discuss the emerging technologies, such as gene therapy, as well as anticipated challenges and their implications in therapeutic development for ciliopathies.

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.

Disruption in murine Eml1 perturbs retinal lamination during early development

During mammalian development, establishing functional neural networks in stratified tissues of the mammalian central nervous system depends upon the proper migration and positioning of neurons, a process known as lamination. In particular, the pseudostratified neuroepithelia of the retina and cerebrocortical ventricular zones provide a platform for progenitor cell proliferation and migration. Lamination defects in these tissues lead to mispositioned neurons, disrupted neuronal connections, and abnormal function. The molecular mechanisms necessary for proper lamination in these tissues are incompletely understood. Here, we identified a nonsense mutation in the Eml1 gene in a novel murine model, tvrm360, displaying subcortical heterotopia, hydrocephalus and disorganization of retinal architecture. In the retina, Eml1 disruption caused abnormal positioning of photoreceptor cell nuclei early in development. Upon maturation, these ectopic photoreceptors possessed cilia and formed synapses but failed to produce robust outer segments, implying a late defect in photoreceptor differentiation secondary to mislocalization. In addition, abnormal positioning of Müller cell bodies and bipolar cells was evident throughout the inner neuroblastic layer. Basal displacement of mitotic nuclei in the retinal neuroepithelium was observed in tvrm360 mice at postnatal day 0. The abnormal positioning of retinal progenitor cells at birth and ectopic presence of photoreceptors and secondary neurons upon maturation suggest that EML1 functions early in eye development and is crucial for proper retinal lamination during cellular proliferation and development.

Splicing in the pathogenesis, diagnosis and treatment of ciliopathies

Primary cilia are essential signalling organelles found on the apical surface of epithelial cells, where they coordinate chemosensation, mechanosensation and light sensation. Motile cilia play a central role in establishing fluid flow in the respiratory tract, reproductive tract, brain ventricles and ear. Genetic defects affecting the structure or function of cilia can lead to a broad range of developmental and degenerative diseases known as ciliopathies. Splicing contributes to the pathogenesis, diagnosis and treatment of ciliopathies. Tissue-specific alternative splicing contributes to the tissue-specific manifestation of ciliopathy phenotypes, for example the retinal-specific effects of some genetic defects, due to specific transcript expression in the highly specialised ciliated cells of the retina, the photoreceptor cells. Ciliopathies can arise both as a result of genetic variants in spliceosomal proteins, or as a result of variants affecting splicing of specific cilia genes. Here we discuss the opportunities and challenges in diagnosing ciliopathies using RNA sequence analysis and the potential for treating ciliopathies in a relatively mutation-neutral way by targeting splicing. This article is part of a Special Issue entitled: RNA structure and splicing regulation edited by Francisco Baralle, Ravindra Singh and Stefan Stamm.

Keeping an Eye on Bardet-Biedl Syndrome: A Comprehensive Review of the Role of Bardet-Biedl Syndrome Genes in the Eye

Upwards of 90% of individuals with Bardet-Biedl syndrome (BBS) display rod-cone dystrophy with early macular involvement. BBS is an autosomal recessive, genetically heterogeneous, pleiotropic ciliopathy for which 21 causative genes have been discovered to date. In addition to retinal degeneration, the cardinal features of BBS include obesity, cognitive impairment, renal anomalies, polydactyly, and hypogonadism. Here, we review the genes, proteins, and protein complexes involved in BBS and the BBS model organisms available for the study of retinal degeneration. We include comprehensive lists for all known BBS genes, their known phenotypes, and the model organisms available. We also review the molecular mechanisms believed to lead to retinal degeneration. We provide an overview of the mode of inheritance and describe the relationships between BBS genes and Joubert syndrome, Leber Congenital Amaurosis, Senior-Løken syndrome, and non-syndromic retinitis pigmentosa. Finally, we propose ways that new advances in technology will allow us to better understand the role of different BBS genes in retinal formation and function.

Functional analysis by minigene assay of putative splicing variants found in Bardet-Biedl syndrome patients

Bardet–Biedl syndrome (BBS) and Alström syndrome (ALMS) are rare diseases belonging to the group of ciliopathies. Although mutational screening studies of BBS/ALMS cohorts have been extensively reported, little is known about the functional effect of those changes. Thus, splicing variants are estimated to represent 15% of disease‐causing mutations, and there is growing evidence that many exonic changes are really splicing variants misclassified. In this study, we aimed to analyse for the first time several variants in BBS2,ARL6/BBS3,BBS4 and ALMS1 genes predicted to produce aberrant splicing by minigene assay. We found discordance between bioinformatics analysis and experimental data when comparing wild‐type and mutant constructs. Remarkably, we identified nonsense variants presumably resistant to nonsense‐mediated decay, even when a premature termination codon would be introduced in the second amino acid (p.(G2*) mutation in ARL6/BBS3 gene). As a whole, we report one of the first functional studies of BBS/ALMS1 variants using minigene assay, trying to elucidate their role in disease. Functional studies of variants identified in BBS and ALMS patients are essential for their proper classification and subsequent genetic counselling and could also be the start point for new therapeutic approaches, currently based only on symptomatic treatment.