Cell-cycle regulation and cell-type specification in the developing Drosophila compound eye

The spatiotemporal order of signaling events unveils the logic of development signaling

Motivation: Animals from worms and insects to birds and mammals show distinct body plans; however, the embryonic development of diverse body plans with tissues and organs within is controlled by a surprisingly few signaling pathways. It is well recognized that combinatorial use of and dynamic interactions among signaling pathways follow specific logic to control complex and accurate developmental signaling and patterning, but it remains elusive what such logic is, or even, what it looks like.

Results: We have developed a computational model for Drosophila eye development with innovated methods to reveal how interactions among multiple pathways control the dynamically generated hexagonal array of R8 cells. We obtained two novel findings. First, the coupling between the long-range inductive signals produced by the proneural Hh signaling and the short-range restrictive signals produced by the antineural Notch and EGFR signaling is essential for generating accurately spaced R8s. Second, the spatiotemporal orders of key signaling events reveal a robust pattern of lateral inhibition conducted by Ato-coordinated Notch and EGFR signaling to collectively determine R8 patterning. This pattern, stipulating the orders of signaling and comparable to the protocols of communication, may help decipher the well-appreciated but poorly defined logic of developmental signaling.

Availability and implementation: The model is available upon request.

Contact:hao.zhu@ymail.com

Supplementary information:Supplementary data are available at Bioinformatics online.

Large scale international replication and meta-analysis study confirms association of the 15q14 locus with myopia. The CREAM consortium

Myopia is a complex genetic disorder and a common cause of visual impairment among working age adults. Genome-wide association studies have identified susceptibility loci on chromosomes 15q14 and 15q25 in Caucasian populations of European ancestry. Here, we present a confirmation and meta-analysis study in which we assessed whether these two loci are also associated with myopia in other populations. The study population comprised 31 cohorts from the Consortium of Refractive Error and Myopia (CREAM) representing 4 different continents with 55,177 individuals; 42,845 Caucasians and 12,332 Asians. We performed a meta-analysis of 14 single nucleotide polymorphisms (SNPs) on 15q14 and 5 SNPs on 15q25 using linear regression analysis with spherical equivalent as a quantitative outcome, adjusted for age and sex. We calculated the odds ratio (OR) of myopia versus hyperopia for carriers of the top-SNP alleles using a fixed effects meta-analysis. At locus 15q14, all SNPs were significantly replicated, with the lowest P value 3.87 × 10(-12) for SNP rs634990 in Caucasians, and 9.65 × 10(-4) for rs8032019 in Asians. The overall meta-analysis provided P value 9.20 × 10(-23) for the top SNP rs634990. The risk of myopia versus hyperopia was OR 1.88 (95 % CI 1.64, 2.16, P < 0.001) for homozygous carriers of the risk allele at the top SNP rs634990, and OR 1.33 (95 % CI 1.19, 1.49, P < 0.001) for heterozygous carriers. SNPs at locus 15q25 did not replicate significantly (P value 5.81 × 10(-2) for top SNP rs939661). We conclude that common variants at chromosome 15q14 influence susceptibility for myopia in Caucasian and Asian populations world-wide.

Specificity of Drosophila cytonemes for distinct signaling pathways

Cytonemes are types of filopodia in the Drosophila wing imaginal disc that are proposed to serve as conduits in which morphogen signaling proteins move between producing and target cells. We investigated the specificity of cytonemes that are made by target cells. Cells in wing discs made cytonemes that responded specifically to Decapentaplegic (Dpp) and cells in eye discs made cytonemes that responded specifically to Spitz (the Drosophila epidermal growth factor protein). Tracheal cells had at least two types: one made in response to Branchless (a Drosophila fibroblast growth factor protein, Bnl), to which they segregate the Bnl receptor, and another to which they segregate the Dpp receptor. We conclude that cells can make several types of cytonemes, each of which responds specifically to a signaling pathway by means of the selective presence of a particular signaling protein receptor that has been localized to that cytoneme.

A genome-wide association study identifies a susceptibility locus for refractive errors and myopia at 15q14

Refractive errors are the most common ocular disorders worldwide and may lead to blindness. Although this trait is highly heritable, identification of susceptibility genes has been challenging. We conducted a genome-wide association study for refractive error in 5,328 individuals from a Dutch population-based study with replication in four independent cohorts (combined 10,280 individuals in the replication stage). We identified a significant association at chromosome 15q14 (rs634990, P = 2.21 × 10⁻¹⁴). The odds ratio of myopia compared to hyperopia for the minor allele (minor allele frequency = 0.47) was 1.41 (95% CI 1.16-1.70) for individuals heterozygous for the allele and 1.83 (95% CI 1.42-2.36) for individuals homozygous for the allele. The associated locus is near two genes that are expressed in the retina, GJD2 and ACTC1, and appears to harbor regulatory elements which may influence transcription of these genes. Our data suggest that common variants at 15q14 influence susceptibility for refractive errors in the general population.

RasGRF1 disruption causes retinal photoreception defects and associated transcriptomic alterations

RasGRF1 null mutant mice display impaired memory/learning and their hippocampus transcriptomic pattern includes a number of differentially expressed genes playing significant roles in sensory development and function. Odour avoidance and auditory brainstem response tests yielded normal results but electroretinographic analysis showed severe light perception impairment in the RasGRF1 knockouts. Whereas no structural alterations distinguished the retinas of wild-type and knockout mice, microarray transcriptional analysis identified at least 44 differentially expressed genes in the retinas of these Knockout animals. Among these, Crb1, Pttg1, Folh1 and Myo7a have been previously related to syndromes involving retina degeneration. Interestingly, over-expression of Folh1 would be expected to result in accumulation of its enzymatic product N-acetyl-aspartate, an event known to be linked to Canavan disease, a human cerebral degenerative syndrome often involving blindness and hearing loss. Consistently, in vivo brain nuclear magnetic resonance spectroscopy identified higher levels of N-acetyl-aspartate in our RasGRF1-/- mice and immunohistochemical analysis detected reduced levels of aspartoacylase, the enzyme which degrades N-acetyl-aspartate. These studies demonstrate for the first time the functional relevance of Ras signalling in mammalian photoreception and warrant further analysis of RasGRF1 Knockout mice as potential models to analyse molecular mechanisms underlying defective photoreception human diseases.

The tumor-suppressor gene fat controls tissue growth upstream of expanded in the hippo signaling pathway

BACKGROUND

The tight control of cell proliferation and cell death is essential to normal tissue development, and the loss of this control is a hallmark of cancers. Cell growth and cell death are coordinately regulated during development by the Hippo signaling pathway. The Hippo pathway consists of the Ste20 family kinase Hippo, the WW adaptor protein Salvador, and the NDR kinase Warts. Loss of Hippo signaling in Drosophila leads to enhanced cell proliferation and decreased apoptosis, resulting in massive tissue overgrowth through increased expression of targets such as Cyclin E and Diap1. The cytoskeletal proteins Merlin and Expanded colocalize at apical junctions and function redundantly upstream of Hippo. It is not clear how they regulate growth or how they are localized to apical junctions.

RESULTS

We find that another Drosophila tumor-suppressor gene, the atypical cadherin fat, regulates both cell proliferation and cell death in developing imaginal discs. Loss of fat leads to increased Cyclin E and Diap1 expression, phenocopying loss of Hippo signaling. Ft can regulate Hippo phosphorylation, a measure of its activation, in tissue culture. Importantly, fat is needed for normal localization of Expanded at apical junctions in vivo. Genetic-epistasis experiments place fat with expanded in the Hippo pathway.

CONCLUSIONS

Together, these data suggest that Fat functions as a cell-surface receptor for the Expanded branch of the conserved Hippo growth control pathway.

The fat cadherin acts through the hippo tumor-suppressor pathway to regulate tissue size

BACKGROUND

The Hippo tumor-suppressor pathway has emerged as a key signaling pathway that controls tissue size in Drosophila. Merlin, the Drosophila homolog of the human Neurofibromatosis type-2 (NF2) tumor-suppressor gene, and the related protein Expanded are the most upstream components of the Hippo pathway identified so far. However, components acting upstream of Expanded and Merlin, such as transmembrane receptors, have not yet been identified.

RESULTS

Here, we report that the protocadherin Fat acts as an upstream component in the Hippo pathway. Fat is a known tumor-suppressor gene in Drosophila, and fat mutants have severely overgrown imaginal discs. We found that the overgrowth phenotypes of fat mutants are similar to those of mutants in Hippo pathway components: fat mutant cells continued to proliferate after wild-type cells stopped proliferating, and fat mutant cells deregulated Hippo target genes such as cyclin E and diap1. Fat acts genetically and biochemically upstream of other Hippo pathway components such as Expanded, the Hippo and Warts kinases, and the transcriptional coactivator Yorkie. Fat is required for the stability of Expanded and its localization to the plasma membrane. In contrast, Fat is not required for Merlin localization, and Fat and Merlin act in parallel in growth regulation.

CONCLUSIONS

Taken together, our data identify a cell-surface molecule that may act as a receptor of the Hippo signaling pathway.

Regulation of early Xenopus development by ErbB signaling

ErbB signaling has long been implicated in cancer formation and progression and is shown to regulate cell division, migration, and death during tumorigenesis. The functions of the ErbB pathway during early vertebrate embryogenesis, however, are not well understood. Here we report characterization of ErbB activities during early frog development. Gain-of-function analyses show that EGFR, ErbB2, and ErbB4 induce ectopic tumor-like cell mass that contains increased numbers of mitotic cells. Both the muscle and the neural markers are expressed in these ectopic protrusions. ErbBs also induce mesodermal markers in ectodermal explants. Loss-of-function studies using carboxyl terminal-truncated dominant-negative ErbB receptors demonstrate that blocking ErbB signals leads to defective gastrulation movements and malformation of the embryonic axis with a reduction in the head structures in early frog embryos. These data, together with the observation that ErbBs are expressed early during frog embryogenesis, suggest that ErbBs regulate cell proliferation, movements, and embryonic patterning during early Xenopus development.