Forward genetics identifies a novel sleep mutant with sleep state inertia and REM sleep deficits.

Switches between global sleep and wakefulness states are believed to be dictated by top-down influences arising from subcortical nuclei. Using forward genetics and in vivo electrophysiology, we identified a recessive mouse mutant line characterized by a substantially reduced propensity to transition between wake and sleep states with an especially pronounced deficit in initiating rapid eye movement (REM) sleep episodes. The causative mutation, an Ile102Asn substitution in the synaptic vesicular protein, VAMP2, was associated with morphological synaptic changes and specific behavioral deficits, while in vitro electrophysiological investigations with fluorescence imaging revealed a markedly diminished probability of vesicular release in mutants. Our data show that global shifts in the synaptic efficiency across brain-wide networks leads to an altered probability of vigilance state transitions, possibly as a result of an altered excitability balance within local circuits controlling sleep-wake architecture.

Ciliary exclusion of Polycystin-2 promotes kidney cystogenesis in an autosomal dominant polycystic kidney disease model.

The human PKD2 locus encodes Polycystin-2 (PC2), a TRPP channel that localises to several distinct cellular compartments, including the cilium. PKD2 mutations cause Autosomal Dominant Polycystic Kidney Disease (ADPKD) and affect many cellular pathways. Data underlining the importance of ciliary PC2 localisation in preventing PKD are limited because PC2 function is ablated throughout the cell in existing model systems. Here, we dissect the ciliary role of PC2 by analysing mice carrying a non-ciliary localising, yet channel-functional, PC2 mutation. Mutants develop embryonic renal cysts that appear indistinguishable from mice completely lacking PC2. Despite not entering the cilium in mutant cells, mutant PC2 accumulates at the ciliary base, forming a ring pattern consistent with distal appendage localisation. This suggests a two-step model of ciliary entry; PC2 first traffics to the cilium base before TOP domain dependent entry. Our results suggest that PC2 localisation to the cilium is necessary to prevent PKD.

Atmin modulates Pkhd1 expression and may mediate Autosomal Recessive Polycystic Kidney Disease (ARPKD) through altered non-canonical Wnt/Planar Cell Polarity (PCP) signalling.

Autosomal Recessive Polycystic Kidney Disease (ARPKD) is a genetic disorder with an incidence of ~1:20,000 that manifests in a wide range of renal and liver disease severity in human patients and can lead to perinatal mortality. ARPKD is caused by mutations in PKHD1, which encodes the large membrane protein, Fibrocystin, required for normal branching morphogenesis of the ureteric bud during embryonic renal development. The variation in ARPKD phenotype suggests that in addition to PKHD1 mutations, other genes may play a role, acting as modifiers of disease severity. One such pathway involves non-canonical Wnt/Planar Cell Polarity (PCP) signalling that has been associated with other cystic kidney diseases, but has not been investigated in ARPKD. Analysis of the AtminGpg6 mouse showed kidney, liver and lung abnormalities, suggesting it as a novel mouse tool for the study of ARPKD. Further, modulation of Atmin affected Pkhd1 mRNA levels, altered non-canonical Wnt/PCP signalling and impacted cellular proliferation and adhesion, although Atmin does not bind directly to the C-terminus of Fibrocystin. Differences in ATMIN and VANGL2 expression were observed between normal human paediatric kidneys and age-matched ARPKD kidneys. Significant increases in ATMIN, WNT5A, VANGL2 and SCRIBBLE were seen in human ARPKD versus normal kidneys; no substantial differences were seen in DAAM2 or NPHP2. A striking increase in E-cadherin was also detected in ARPKD kidneys. This work indicates a novel role for non-canonical Wnt/PCP signalling in ARPKD and suggests ATMIN as a modulator of PKHD1.

A Wars2 Mutant Mouse Model Displays OXPHOS Deficiencies and Activation of Tissue-Specific Stress Response Pathways.

Mutations in genes essential for mitochondrial function have pleiotropic effects. The mechanisms underlying these traits yield insights into metabolic homeostasis and potential therapies. Here we report the characterization of a mouse model harboring a mutation in the tryptophanyl-tRNA synthetase 2 (Wars2) gene, encoding the mitochondrial-localized WARS2 protein. This hypomorphic allele causes progressive tissue-specific pathologies, including hearing loss, reduced adiposity, adipose tissue dysfunction, and hypertrophic cardiomyopathy. We demonstrate the tissue heterogeneity arises as a result of variable activation of the integrated stress response (ISR) pathway and the ability of certain tissues to respond to impaired mitochondrial translation. Many of the systemic metabolic effects are likely mediated through elevated fibroblast growth factor 21 (FGF21) following activation of the ISR in certain tissues. These findings demonstrate the potential pleiotropy associated with Wars2 mutations in patients.

Analysis of mammalian gene function through broad-based phenotypic screens across a consortium of mouse clinics.

The function of the majority of genes in the mouse and human genomes remains unknown. The mouse embryonic stem cell knockout resource provides a basis for the characterization of relationships between genes and phenotypes. The EUMODIC consortium developed and validated robust methodologies for the broad-based phenotyping of knockouts through a pipeline comprising 20 disease-oriented platforms. We developed new statistical methods for pipeline design and data analysis aimed at detecting reproducible phenotypes with high power. We acquired phenotype data from 449 mutant alleles, representing 320 unique genes, of which half had no previous functional annotation. We captured data from over 27,000 mice, finding that 83% of the mutant lines are phenodeviant, with 65% demonstrating pleiotropy. Surprisingly, we found significant differences in phenotype annotation according to zygosity. New phenotypes were uncovered for many genes with previously unknown function, providing a powerful basis for hypothesis generation and further investigation in diverse systems.

ATMIN is a transcriptional regulator of both lung morphogenesis and ciliogenesis.

Initially identified in DNA damage repair, ATM-interactor (ATMIN) further functions as a transcriptional regulator of lung morphogenesis. Here we analyse three mouse mutants, Atmin(gpg6/gpg6), Atmin(H210Q/H210Q) and Dynll1(GT/GT), revealing how ATMIN and its transcriptional target dynein light chain LC8-type 1 (DYNLL1) are required for normal lung morphogenesis and ciliogenesis. Expression screening of ciliogenic genes confirmed Dynll1 to be controlled by ATMIN and further revealed moderately altered expression of known intraflagellar transport (IFT) protein-encoding loci in Atmin mutant embryos. Significantly, Dynll1(GT/GT) embryonic cilia exhibited shortening and bulging, highly similar to the characterised retrograde IFT phenotype of Dync2h1. Depletion of ATMIN or DYNLL1 in cultured cells recapitulated the in vivo ciliogenesis phenotypes and expression of DYNLL1 or the related DYNLL2 rescued the effects of loss of ATMIN, demonstrating that ATMIN primarily promotes ciliogenesis by regulating Dynll1 expression. Furthermore, DYNLL1 as well as DYNLL2 localised to cilia in puncta, consistent with IFT particles, and physically interacted with WDR34, a mammalian homologue of the Chlamydomonas cytoplasmic dynein 2 intermediate chain that also localised to the cilium. This study extends the established Atmin-Dynll1 relationship into a developmental and a ciliary context, uncovering a novel series of interactions between DYNLL1, WDR34 and ATMIN. This identifies potential novel components of cytoplasmic dynein 2 and furthermore provides fresh insights into the molecular pathogenesis of human skeletal ciliopathies.

A novel mouse Fgfr2 mutant, hobbyhorse (hob), exhibits complete XY gonadal sex reversal.

The secreted molecule fibroblast growth factor 9 (FGF9) plays a critical role in testis determination in the mouse. In embryonic gonadal somatic cells it is required for maintenance of SOX9 expression, a key determinant of Sertoli cell fate. Conditional gene targeting studies have identified FGFR2 as the main gonadal receptor for FGF9 during sex determination. However, such studies can be complicated by inefficient and variable deletion of floxed alleles, depending on the choice of Cre deleter strain. Here, we report a novel, constitutive allele of Fgfr2, hobbyhorse (hob), which was identified in an ENU-based forward genetic screen for novel testis-determining loci. Fgr2hob is caused by a C to T mutation in the invariant exon 7, resulting in a polypeptide with a mis-sense mutation at position 263 (Pro263Ser) in the third extracellular immunoglobulin-like domain of FGFR2. Mutant homozygous embryos show severe limb and lung defects and, when on the sensitised C57BL/6J (B6) genetic background, undergo complete XY gonadal sex reversal associated with failure to maintain expression of Sox9. Genetic crosses employing a null mutant of Fgfr2 suggest that Fgr2hob is a hypomorphic allele, affecting both the FGFR2b and FGFR2c splice isoforms of the receptor. We exploited the consistent phenotype of this constitutive mutant by analysing MAPK signalling at the sex-determining stage of gonad development, but no significant abnormalities in mutant embryos were detected.

Dominant ß-catenin mutations cause intellectual disability with recognizable syndromic features.

The recent identification of multiple dominant mutations in the gene encoding ß-catenin in both humans and mice has enabled exploration of the molecular and cellular basis of ß-catenin function in cognitive impairment. In humans, ß-catenin mutations that cause a spectrum of neurodevelopmental disorders have been identified. We identified de novo ß-catenin mutations in patients with intellectual disability, carefully characterized their phenotypes, and were able to define a recognizable intellectual disability syndrome. In parallel, characterization of a chemically mutagenized mouse line that displays features similar to those of human patients with ß-catenin mutations enabled us to investigate the consequences of ß-catenin dysfunction through development and into adulthood. The mouse mutant, designated batface (Bfc), carries a Thr653Lys substitution in the C-terminal armadillo repeat of ß-catenin and displayed a reduced affinity for membrane-associated cadherins. In association with this decreased cadherin interaction, we found that the mutation results in decreased intrahemispheric connections, with deficits in dendritic branching, long-term potentiation, and cognitive function. Our study provides in vivo evidence that dominant mutations in ß-catenin underlie losses in its adhesion-related functions, which leads to severe consequences, including intellectual disability, childhood hypotonia, progressive spasticity of lower limbs, and abnormal craniofacial features in adults.

Genome-wide association study using extreme truncate selection identifies novel genes affecting bone mineral density and fracture risk.

Osteoporotic fracture is a major cause of morbidity and mortality worldwide. Low bone mineral density (BMD) is a major predisposing factor to fracture and is known to be highly heritable. Site-, gender-, and age-specific genetic effects on BMD are thought to be significant, but have largely not been considered in the design of genome-wide association studies (GWAS) of BMD to date. We report here a GWAS using a novel study design focusing on women of a specific age (postmenopausal women, age 55-85 years), with either extreme high or low hip BMD (age- and gender-adjusted BMD z-scores of +1.5 to +4.0, n = 1055, or -4.0 to -1.5, n = 900), with replication in cohorts of women drawn from the general population (n = 20,898). The study replicates 21 of 26 known BMD-associated genes. Additionally, we report suggestive association of a further six new genetic associations in or around the genes CLCN7, GALNT3, IBSP, LTBP3, RSPO3, and SOX4, with replication in two independent datasets. A novel mouse model with a loss-of-function mutation in GALNT3 is also reported, which has high bone mass, supporting the involvement of this gene in BMD determination. In addition to identifying further genes associated with BMD, this study confirms the efficiency of extreme-truncate selection designs for quantitative trait association studies.

Dystroglycan regulates structure, proliferation and differentiation of neuroepithelial cells in the developing vertebrate CNS.

In the developing CNS alpha- and beta-dystroglycan are highly concentrated in the endfeet of radial neuroepithelial cells at the contact site to the basal lamina. We show that injection of anti-dystroglycan Fab fragments, knockdown of dystroglycan using RNAi, and overexpression of a dominant-negative dystroglycan protein by microelectroporation in neuroepithelial cells of the chick retina and optic tectum in vivo leads to the loss of their radial morphology, to hyperproliferation, to an increased number of postmitotic neurons, and to an altered distribution of several basally concentrated proteins. Moreover, these treatments also altered the oriented growth of axons from retinal ganglion cells and from tectal projection neurons. In contrast, expression of non-cleavable dystroglycan protein in neuroepithelial cells reduced their proliferation and their differentiation to postmitotic neurons. These results demonstrate that dystroglycan plays a key role in maintaining neuroepithelial cell morphology, and that interfering with dystroglycan function influences proliferation and differentiation of neuroepithelial cells. These data also suggest that an impaired dystroglycan function in neuroepithelial cells might be responsible for some of the severe brain abnormalities observed in certain forms of congenital muscular dystrophy.

The effects of post-translational processing on dystroglycan synthesis and trafficking.

Dystroglycan is a component of the dystrophin glycoprotein complex that is cleaved into two polypeptides by an unidentified protease. To determine the role of post-translational processing on dystroglycan synthesis and trafficking we expressed the dystroglycan precursor and mutants thereof in a heterologous system. A point mutant in the processing site, S655A, prevented proteolytic cleavage but had no effect upon the surface localisation of dystroglycan. Mutation of two N-linked glycosylation sites that flank the cleavage site inhibited proteolytic processing of the precursor. Furthermore, chemical inhibition of N- and O-linked glycosylation interfered with the processing of the precursor and reduced the levels of dystroglycan at the cell surface. Dystroglycan processing was also inhibited by the proteasome inhibitor lactacystin. N-linked glycosylation is a prerequisite for efficient proteolytic processing and cleavage and glycosylation are dispensable for cell surface targeting of dystroglycan.

Functional requirements for fukutin-related protein in the Golgi apparatus.

Two forms of congenital muscular dystrophy (CMD), Fukuyama CMD and CMD type 1C (MDC1C) are caused by mutations in the genes encoding two putative glycosyltransferases, fukutin and fukutin-related protein (FKRP). Additionally, mutations in the FKRP gene also cause limb-girdle muscular dystrophy type 2I (LGMD2I), a considerably milder allelic variant than MDC1C. All of these diseases are associated with secondary changes in muscle alpha-dystroglycan expression. To elucidate the function of FKRP and fukutin and examine the effects of MDC1C patient mutations, we have determined the mechanism for the subcellular location of each protein. FKRP and fukutin are targeted to the medial-Golgi apparatus through their N-termini and transmembrane domains. Overexpression of FKRP in CHO cells alters the post-translational processing of alpha- and beta-dystroglycan inhibiting maturation of the two isoforms. Mutations in the DxD motif in the putative active site of the protein or in the Golgi-targeting sequence, which cause FKRP to be inefficiently trafficked to the Golgi apparatus, did not alter dystroglycan processing in vitro. The P448L mutation in FKRP that causes congenital muscular dystrophy changes a conserved amino acid resulting in the mislocalization of the mutant protein in the cell that is unable to alter dystroglycan processing. Our data show that FKRP and fukutin are Golgi-resident proteins and that FKRP is required for the post-translational modification of dystroglycan. Aberrant processing of dystroglycan caused by a mislocalized FKRP mutant could be a novel mechanism that causes congenital muscular dystrophy.