Mutations in CEP290, a large multidomain coiled coil protein, are associated with multiple cilia-associated syndromes. Over 130 CEP290 mutations have been linked to a wide spectrum of human ciliopathies, raising the question of how mutations in a single gene cause different disease syndromes. In zebrafish the expressivity of cep290 deficiencies were linked to the type of genetic ablation: acute cep290 morpholino knockdown caused severe cilia-related phenotypes while defects in a Crispr/Cas9 genetic mutant were restricted to photoreceptor defects. Here we show that milder phenotypes in genetic mutants were associated with upregulation of genes encoding the cilia-associated small GTPases arl3, arl13b, and unc119b. Upregulation of UNC119b was also observed in urine-derived renal epithelial cells from human JBTS CEP290 patients. Ectopic expression of arl3, arl13b and unc119b in cep290 morphant zebrafish embryos rescued Kupffer's vesicle cilia and partially rescued photoreceptor outer segment defects. The results suggest that genetic compensation by upregulation of genes involved in a common subcellular process, lipidated protein trafficking to cilia, may be a conserved mechanism contributing to genotype-phenotype variations observed in CEP290 deficiencies.

Numerous genes are involved in human growth regulation. Recently, autosomal-recessive inherited variants in centrosomal proteins have been identified in Seckel syndrome, primary microcephaly, or microcephalic osteodysplastic primary dwarfism. Common hallmarks of these syndromic forms are severe short stature and microcephaly. In a consanguineous family with two affected children with severe growth retardation and normocephaly, we used homozygosity mapping and next-generation sequencing to identify a homozygous MAP4 variant. MAP4 is a major protein for microtubule assembly during mitosis. High-expression levels in the somite boundaries of zebrafish suggested a role in growth and body segment patterning. The identified variant affects binding sites of kinases necessary for dynamic instability of microtubule formation. We found centrosome amplifications in mitotic fibroblast cells in vivo and in vitro. These numeric centrosomal aberrations were also present during interphase resulting in aberrant ciliogenesis. Furthermore, affected cells showed a dysfunction of the microtubule-dependent assembly of the Golgi apparatus, indicated by a significant lack of compactness of Golgi membranes. These observations demonstrated that MAP4 mutations contribute to the clinical spectrum of centrosomal defects and confirmed the complex role of a centrosomal protein in centrosomal, ciliary, and Golgi regulation associated with severe short stature.

Podocyte dysfunction impairs the size selectivity of the glomerular filter, leading to proteinuria, hypoalbuminuria, and edema, clinically defined as nephrotic syndrome. Hereditary forms of nephrotic syndrome are linked to mutations in podocyte-specific genes. To identify genes contributing to podocyte dysfunction in acquired nephrotic syndrome, we studied human glomerular gene expression data sets for glomerular-enriched gene transcripts differentially regulated between pretransplant biopsy samples and biopsies from patients with nephrotic syndrome. Candidate genes were screened by in situ hybridization for expression in the zebrafish pronephros, an easy-to-use in vivo assay system to assess podocyte function. One glomerulus-enriched product was the Rho-GTPase binding protein, IQGAP2. Immunohistochemistry found a strong presence of IQGAP2 in normal human and zebrafish podocytes. In zebrafish larvae, morpholino-based knockdown of iqgap2 caused a mild foot process effacement of zebrafish podocytes and a cystic dilation of the urinary space of Bowman's capsule upon onset of urinary filtration. Moreover, the glomerulus of zebrafish morphants showed a glomerular permeability for injected high-molecular-weight dextrans, indicating an impaired size selectivity of the glomerular filter. Thus, IQGAP2 is a Rho-GTPase binding protein, highly abundant in human and zebrafish podocytes, which controls normal podocyte structure and function as evidenced in the zebrafish pronephros.

Klotho is an aging suppressor gene. In mice, loss of Klotho function causes accelerated aging while increased Klotho expression increases longevity. This study aimed to identify and characterize the orthologue of Klotho in zebrafish, a powerful model organism for the investigation of development and human disease. Zebrafish klotho was identified by a bioinformatics approach, and cloning and sequencing of klotho cDNA confirmed the in silico analysis. The zebrafish Klotho protein has a structure similar to human and mouse Klotho, but it lacks an apparent secretory signal sequence. We can find no evidence of an alternative transcript isoform lacking the transmembrane domain coding sequence as seen in mammals. RT-PCR revealed the expression of klotho during embryonic development and in a wider variety of adult tissues than in mouse. Quantitative real-time RT-PCR demonstrated the relative gene expression profile of zebrafish Klotho during embryogenesis and in adult tissues. In situ hybridization showed an apparently diffuse signal of klotho mRNA expression in the adult zebrafish testis.