Novel regulators of planar cell polarity: a genetic analysis in Drosophila

The Nab2 RNA-binding protein patterns dendritic and axonal projections through a planar cell polarity-sensitive mechanism

RNA-binding proteins support neurodevelopment by modulating numerous steps in post-transcriptional regulation, including splicing, export, translation, and turnover of mRNAs that can traffic into axons and dendrites. One such RNA-binding protein is ZC3H14, which is lost in an inherited intellectual disability. The Drosophila melanogaster ZC3H14 ortholog, Nab2, localizes to neuronal nuclei and cytoplasmic ribonucleoprotein granules and is required for olfactory memory and proper axon projection into brain mushroom bodies. Nab2 can act as a translational repressor in conjunction with the Fragile-X mental retardation protein homolog Fmr1 and shares target RNAs with the Fmr1-interacting RNA-binding protein Ataxin-2. However, neuronal signaling pathways regulated by Nab2 and their potential roles outside of mushroom body axons remain undefined. Here, we present an analysis of a brain proteomic dataset that indicates that multiple planar cell polarity proteins are affected by Nab2 loss, and couple this with genetic data that demonstrate that Nab2 has a previously unappreciated role in restricting the growth and branching of dendrites that elaborate from larval body-wall sensory neurons. Further analysis confirms that Nab2 loss sensitizes sensory dendrites to the genetic dose of planar cell polarity components and that Nab2-planar cell polarity genetic interactions are also observed during Nab2-dependent control of axon projection in the central nervous system mushroom bodies. Collectively, these data identify the conserved Nab2 RNA-binding protein as a likely component of post-transcriptional mechanisms that limit dendrite growth and branching in Drosophila sensory neurons and genetically link this role to the planar cell polarity pathway. Given that mammalian ZC3H14 localizes to dendritic spines and controls spine density in hippocampal neurons, these Nab2-planar cell polarity genetic data may highlight a conserved path through which Nab2/ZC3H14 loss affects morphogenesis of both axons and dendrites in diverse species.

AKAP200 promotes Notch stability by protecting it from Cbl/lysosome-mediated degradation in Drosophila melanogaster

AKAP200 is a Drosophila melanogaster member of the “A Kinase Associated Protein” family of scaffolding proteins, known for their role in the spatial and temporal regulation of Protein Kinase A (PKA) in multiple signaling contexts. Here, we demonstrate an unexpected function of AKAP200 in promoting Notch protein stability. In Drosophila, AKAP200 loss-of-function (LOF) mutants show phenotypes that resemble Notch LOF defects, including eye patterning and sensory organ specification defects. Through genetic interactions, we demonstrate that AKAP200 interacts positively with Notch in both the eye and the thorax. We further show that AKAP200 is part of a physical complex with Notch. Biochemical studies reveal that AKAP200 stabilizes endogenous Notch protein, and that it limits ubiquitination of Notch. Specifically, our genetic and biochemical evidence indicates that AKAP200 protects Notch from the E3-ubiquitin ligase Cbl, which targets Notch to the lysosomal pathway. Indeed, we demonstrate that the effect of AKAP200 on Notch levels depends on the lysosome. Interestingly, this function of AKAP200 is fully independent of its role in PKA signaling and independent of its ability to bind PKA. Taken together, our data indicate that AKAP200 is a novel tissue specific posttranslational regulator of Notch, maintaining high Notch protein levels and thus promoting Notch signaling.

AKAP200 belongs to a family of scaffolding proteins best known for their regulation of PKA localization. In this study, we have identified a novel role of AKAP200 in Notch protein stability and signaling. In Drosophila melanogaster, AKAP200’s loss and gain-of-function (LOF/GOF) phenotypes are characteristic of Notch signaling defects. Furthermore, we demonstrated genetic interactions between AKAP200 and Notch. Consistent with this, AKAP200 stabilizes the endogenous Notch protein and limits its ubiquitination. AKAP200 exerts its effects on Notch by antagonizing Cbl-mediated ubiquitination and thus lysosome targeting of Notch. Based on these data, we postulate a novel PKA independent mechanism of AKAP200 to achieve optimal Notch protein levels, with AKAP200 preventing Cbl-mediated lysosomal degradation of Notch.

Leger RJ. The genetic basis for variation in resistance to infection in the Drosophila melanogaster genetic reference panel

Individuals vary extensively in the way they respond to disease but the genetic basis of this variation is not fully understood. We found substantial individual variation in resistance and tolerance to the fungal pathogen Metarhizium anisopliae Ma549 using the Drosophila melanogaster Genetic Reference Panel (DGRP). In addition, we found that host defense to Ma549 was correlated with defense to the bacterium Pseudomonas aeruginosa Pa14, and several previously published DGRP phenotypes including oxidative stress sensitivity, starvation stress resistance, hemolymph glucose levels, and sleep indices. We identified polymorphisms associated with differences between lines in both their mean survival times and microenvironmental plasticity, suggesting that lines differ in their ability to adapt to variable pathogen exposures. The majority of polymorphisms increasing resistance to Ma549 were sex biased, located in non-coding regions, had moderately large effect and were rare, suggesting that there is a general cost to defense. Nevertheless, host defense was not negatively correlated with overall longevity and fecundity. In contrast to Ma549, minor alleles were concentrated in the most Pa14-susceptible as well as the most Pa14-resistant lines. A pathway based analysis revealed a network of Pa14 and Ma549-resistance genes that are functionally connected through processes that encompass phagocytosis and engulfment, cell mobility, intermediary metabolism, protein phosphorylation, axon guidance, response to DNA damage, and drug metabolism. Functional testing with insertional mutagenesis lines indicates that 12/13 candidate genes tested influence susceptibility to Ma549. Many candidate genes have homologs identified in studies of human disease, suggesting that genes affecting variation in susceptibility are conserved across species.

APC/CFzr/Cdh1-Dependent Regulation of Planar Cell Polarity Establishment via Nek2 Kinase Acting on Dishevelled

The Anaphase-Promoting Complex/Cyclosome (APC/C) is an E3 ubiquitin ligase, well known for its role in cell-cycle progression. However, it has been linked to additional functions, mainly in neuronal contexts, when using the co-activator Cdh1/Fzr. Here, our data indicate a post-mitotic requirement for the APC/C^Fzr/Cdh1 in epithelial cell patterning and planar cell polarity (PCP) in Drosophila. PCP signaling is critical for development by establishing cellular asymmetries and orientation within the plane of an epithelium, via differential localization of distinct complexes of core PCP factors. Loss of APC/C function leads to reduced levels of Dishevelled (Dsh), a core PCP factor. The effect of APC/C on Dsh is mediated by Nek2 kinase, which can phosphorylate Dsh and is a direct APC/C^Fzr/Cdh1 substrate. We have thus uncovered a pathway of regulation whereby APC/C^Fzr/Cdh1 negatively regulates Nek2, which negatively regulates Dsh, to ensure its proper stoichiometric requirement and localization during PCP establishment.

A Novel Frizzled-Based Screening Tool Identifies Genetic Modifiers of Planar Cell Polarity in Drosophila Wings

Most mutant alleles in the Fz-PCP pathway genes were discovered in classic Drosophila screens looking for recessive loss-of-function (LOF) mutations. Nonetheless, although Fz-PCP signaling is sensitive to increased doses of PCP gene products, not many screens have been performed in the wing under genetically engineered Fz overexpression conditions, mostly because the Fz phenotypes were strong and/or not easy to score and quantify. Here, we present a screen based on an unexpected mild Frizzled gain-of-function (GOF) phenotype. The leakiness of a chimeric Frizzled protein designed to be accumulated in the endoplasmic reticulum (ER) generated a reproducible Frizzled GOF phenotype in Drosophila wings. Using this genotype, we first screened a genome-wide collection of large deficiencies and found 16 strongly interacting genomic regions. Next, we narrowed down seven of those regions to finally test 116 candidate genes. We were, thus, able to identify eight new loci with a potential function in the PCP context. We further analyzed and confirmed krasavietz and its interactor short-stop as new genes acting during planar cell polarity establishment with a function related to actin and microtubule dynamics.

The Dishevelled Protein Family: Still Rather a Mystery After Over 20 Years of Molecular Studies

Dishevelled (Dsh) is a key component of Wnt-signaling pathways and possibly also has other functional requirements. Dsh appears to be a key factor to interpret Wnt signals coming via the Wnt-receptor family, the Frizzled proteins, from the plasma membrane and route them into the correct intracellular pathways. However, how Dsh is regulated to relay signal flow to specific and distinct cellular responses upon interaction with the same Wnt-receptor family remains very poorly understood.

The clathrin adaptor AP-1 complex and Arf1 regulate planar cell polarity in vivo

A key step in generating planar cell polarity (PCP) is the formation of restricted junctional domains containing Frizzled/Dishevelled/Diego (Fz/Dsh/Dgo) or Van Gogh/Prickle (Vang/Pk) complexes within the same cell, stabilized via Flamingo (Fmi) across cell membranes. Although models have been proposed for how these complexes acquire and maintain their polarized localization, the machinery involved in moving core PCP proteins around cells remains unknown. We describe the AP-1 adaptor complex and Arf1 as major regulators of PCP protein trafficking in vivo. AP-1 and Arf1 disruption affects the accumulation of Fz/Fmi and Vang/Fmi complexes in the proximo-distal axis, producing severe PCP phenotypes. Using novel tools, we demonstrate a direct and specific Arf1 involvement in Fz trafficking in vivo. Moreover, we uncover a conserved Arf1 PCP function in vertebrates. Our data support a model whereby the trafficking machinery plays an important part during PCP establishment, promoting formation of polarized PCP-core complexes in vivo.

Nemo regulates cell dynamics and represses the expression of miple, a midkine/pleiotrophin cytokine, during ommatidial rotation

Ommatidial rotation is one of the most important events for correct patterning of the Drosophila eye. Although several signaling pathways are involved in this process, few genes have been shown to specifically affect it. One of them is nemo (nmo), which encodes a MAP-like protein kinase that regulates the rate of rotation throughout the entire process, and serves as a link between core planar cell polarity (PCP) factors and the E-cadherin-β-catenin complex. To determine more precisely the role of nmo in ommatidial rotation, live-imaging analyses in nmo mutant and wild-type early pupal eye discs were performed. We demonstrate that ommatidial rotation is not a continuous process, and that rotating and non-rotating interommatidial cells are very dynamic. Our in vivo analyses also show that nmo regulates the speed of rotation and is required in cone cells for correct ommatidial rotation, and that these cells as well as interommatidial cells are less dynamic in nmo mutants. Furthermore, microarray analyses of nmo and wild-type larval eye discs led us to identify new genes and signaling pathways related to nmo function during this process. One of them, miple, encodes the Drosophila ortholog of the midkine/pleiotrophin secreted cytokines that are involved in cell migration processes. miple is highly up-regulated in nmo mutant discs. Indeed, phenotypic analyses reveal that miple overexpression leads to ommatidial rotation defects. Genetic interaction assays suggest that miple is signaling through Ptp99A, the Drosophila ortholog of the vertebrate midkine/pleiotrophin PTPζ receptor. Accordingly, we propose that one of the roles of Nmo during ommatial rotation is to repress miple expression, which may in turn affect the dynamics in E-cadherin-β-catenin complexes.