Bardet–Biedl syndrome type 4 (BBS4)-null mice implicate Bbs4 in flagella formation but not global cilia assembly

Abstract

The functions of the proteins encoded by the Bardet-Biedl syndrome (BBS) genes are unknown. Mutations in these genes lead to the pleiotropic human disorder BBS, which is characterized by obesity, retinopathy, polydactyly, renal and cardiac malformations, learning disabilities, and hypogenitalism. Secondary features include diabetes mellitus and hypertension. Recently, it has been suggested that the BBS phenotypes are the result of a lack of cilia formation or function. In this study, we show that mice lacking the Bbs4 protein have major components of the human phenotype, including obesity and retinal degeneration. We show that Bbs4-null mice develop both motile and primary cilia, demonstrating that Bbs4 is not required for global cilia formation. Interestingly, male Bbs4-null mice do not form spermatozoa flagella, and BBS4 retinopathy involves apoptotic death of photoreceptors, the primary ciliated cells of the retina. These mutation data demonstrate a connection between the function of a BBS protein and cilia. To further evaluate an association between cilia and BBS, we performed homology comparisons of BBS proteins in model organisms and find that BBS proteins are specifically conserved in ciliated organisms.B ardet-Biedl syndrome (BBS) is a genetically heterogeneous disorder with linkage to eight loci. Six BBS genes (BBS1, BBS2, BBS4, BBS6, BBS7, and BBS8) have been identified (1-7). With the exception of BBS6, which has similarity to type II chaperonins (8), BBS proteins do not show extensive homology with proteins of known function. BBS4 and BBS8 contain tetratricopeptide repeat domains, and a region of BBS8 shows similarity to the prokaryotic pilF domain. Based on the observations that BBS8 localizes to the basal body of ciliated cells and expression of the Caenorhabditis elegans orthologues of several BBS proteins are limited to ciliated cells, it has been hypothesized that BBS is the result of a defect in cilia assembly or function (7). We previously used positional cloning to identify the human BBS4 gene (3). To help elucidate the function of the BBS proteins, we have now targeted the Bbs4 gene in mice. Bbs4 Ϫ/Ϫ mice recapitulate aspects of the human phenotype: they become obese, fail to reproduce, and display retinal degeneration. We show the presence of both motile and primary cilia in these mice, demonstrating that Bbs4 deficiency does not prevent global ciliogenesis. Interestingly, male knockout mice have a complete lack of flagella, demonstrating that the Bbs4 protein is necessary for flagella formation during spermatogenesis. Furthermore, Bbs4-null mice undergo retinal degeneration due principally to photoreceptor cell loss associated with outer segment attenuation, suggesting a role for BBS proteins in maintenance of sensory cilia. These data demonstrate a role for BBS proteins in facets of cilia function. Materials and MethodsConstruction of the Bbs4 Gene Targeting Vector. BLAST analysis of the Celera mouse fragment database with the human BBS4 cDNA sequence identified sequences corresponding to...

Homozygosity mapping with SNP arrays identifiesTRIM32, an E3 ubiquitin ligase, as a Bardet–Biedl syndrome gene (BBS11)

“…Second, there are three relevant knockout mouse models for genes within the linked interval: Pappa (31), Astn1 (paralogue of Astn2) (32), and Tlr4 (33). These models do not have phenotypes that resemble BBS mouse models (34)(35)(36). Finally, functional characterization of other TRIM proteins indicates involvement with components of the cytoskeleton, a finding consistent with the function of other BBS proteins (37)(38)(39).…”

Cilium-generated signaling and cilia-related disorders

“…Since obesity and several of the other phenotypes that accompany BBS are common, studying this disorder potentially will yield new insights into the cellular and molecular mechanisms that underlie these important phenotypes. That several of the disease phenotypes (retinal dystrophy, situs inversus, and anosmia) already were linked to cilia suggested a connection between BBS and cilia, and several recent studies have confirmed this idea [94][95][96][97] (reviewed in Mykytyn and Sheffield 98 ). The BBS1 and BBS4 gene products are essential for formation of normal cilia in mouse olfactory epithelium.…”

“…97 Interestingly, BBS4 knockout mice also undergo apoptotic retinal degeneration and males are sterile because their sperm fail to form a flagellum. 95,96 On the other hand, the BBS4 mice (which exhibit anosmia) show normal motile cilia in the airway and respiratory epithelial, as well as cilia of normal appearance in the renal epithelium. Thus, BBS4 protein is not globally required for cilia formation.…”

Ciliary dysfunction impairs beta-cell insulin secretion and promotes development of type 2 diabetes in rodents

“…At birth, Bardet-Biedl-Syndrome 4 (Bbs4) mutant mice are runted compared with their wild-type littermates and manifest obesity later on 15 . Penetrance is dependent on sex and age of the animals both in this and another Bbs4 À / À mutant strain 16 . Generally, female mutants are more severely affected than age-matched male mutants.…”