The Drosophila anterior-posterior axis is polarized by asymmetric myosin activation

The Drosophila anterior-posterior axis is specified at mid-oogenesis when the Par-1 kinase is recruited to the posterior cortex of the oocyte, where it polarizes the microtubule cytoskeleton to define where the axis determinants, bicoid and oskar mRNAs, localize. This polarity is established in response to an unknown signal from the follicle cells, but how this occurs is unclear. Here we show that the myosin chaperone Unc-45 and non-muscle myosin II (MyoII) are required upstream of Par-1 in polarity establishment. Furthermore, the myosin regulatory light chain (MRLC) is di-phosphorylated at the oocyte posterior in response to the follicle cell signal, inducing longer pulses of myosin contractility at the posterior that may increase cortical tension. Overexpression of MRLC-T21A that cannot be di-phosphorylated or treatment with the myosin light-chain kinase inhibitor ML-7 abolishes Par-1 localization, indicating that the posterior of MRLC di-phosphorylation is essential for both polarity establishment and maintenance. Thus, asymmetric myosin activation polarizes the anterior-posterior axis by recruiting and maintaining Par-1 at the posterior cortex. This raises an intriguing parallel with anterior-posterior axis formation in C. elegans, where MyoII also acts upstream of the PAR proteins to establish polarity, but to localize the anterior PAR proteins rather than Par-1.

Fat-regulated adaptor protein Dlish binds the growth suppressor Expanded and controls its stability and ubiquitination

To regulate the growth and size of organs, cells can use information from their neighbors to modify intracellular mediators of cell proliferation. The intracellular Hippo pathway is a widely utilized nexus for growth control in animals, but its regulation by extracellular signals is not fully understood. We here identify a pathway that regulates organ size in Drosophila, triggered by the transmembrane receptor, the giant protocadherin Fat. We show that the Fat-regulated SH3 domain adaptor protein Dlish binds to and reduces the stability of the growth suppressor Expanded, a known regulator of the Hippo pathway. The destabilization of Expanded by Dlish works in parallel to a previously established pathway in which Dlish increases levels of the growth-stimulating protein Dachs.

The Drosophila protocadherin Fat controls organ size through the Hippo pathway, but the biochemical links to the Hippo pathway components are still poorly defined. We previously identified Dlish, an SH3 domain protein that physically interacts with Fat and the type XX myosin Dachs, and showed that Fat’s regulation of Dlish levels and activity helps limit Dachs-mediated inhibition of Hippo pathway activity. We here characterize a parallel growth control pathway downstream of Fat and Dlish. Using immunoprecipitation and mass spectrometry to search for Dlish partners, we find that Dlish binds the FERM domain growth repressor Expanded (Ex); Dlish SH3 domains directly bind sites in the Ex C terminus. We further show that, in vivo, Dlish reduces the subapical accumulation of Ex, and that loss of Dlish blocks the destabilization of Ex caused by loss of Fat. Moreover, Dlish can bind the F-box E3 ubiquitin ligase Slimb and promote Slimb-mediated ubiquitination of Expanded in vitro. Both the in vitro and in vivo effects of Dlish on Ex require Slimb, strongly suggesting that Dlish destabilizes Ex by helping recruit Slimb-containing E3 ubiquitin ligase complexes to Ex.

Proteostasis in the Hedgehog signaling pathway

The Hedgehog (Hh) signaling pathway is crucial for the development of vertebrate and invertebrate animals alike. Hh ligand binds its receptor Patched (Ptc), allowing the activation of the obligate signal transducer Smoothened (Smo). The levels and localizations of both Ptc and Smo are regulated by ubiquitination, and Smo is under additional regulation by phosphorylation and SUMOylation. Downstream of Smo, the Ci/Gli family of transcription factors regulates the transcriptional responses to Hh. Phosphorylation, ubiquitination and SUMOylation are important for the stability and localization of Ci/Gli proteins and Hh signaling output. Finally, Suppressor of Fused directly regulates Ci/Gli proteins and itself is under proteolytic regulation that is critical for normal Hh signaling.

The Drosophila F-box protein Slimb controls dSmurf protein turnover to regulate the Hippo pathway

SMAD ubiquitination regulatory factors 1 and 2 (Smurf1/2) are members of the HECT domain E3 ligase family which play crucial roles in the regulation of cell cycle progression, planar cell polarity, cancer metastasis and cell apoptosis. We recently showed that the Drosophila homolog dSmurf controls the stability of Warts kinase to regulate the Hippo pathway. In the current study, we found that the F-box protein Slimb controls dSmurf protein level to regulate the Hippo pathway. Slimb physically associates with dSmurf as revealed by co-immunoprecipitation assay in S2 cells. The C-terminal WD40 repeats of Slimb (188-510 amino acid) and the C-terminal HECT domain of dSmurf (723-1061 amino acid) are necessary for their binding. Interaction with Slimb leads to the ubiquitination and degradation of dSmurf, resulting in negative regulation of dSmurf-mediated Yki phosphorylation and activity in the Hippo pathway. Thus our study revealed a new regulatory mechanism of the Hippo pathway which may provide implications for developing tumor treatment.

Asterless Reduction during Spermiogenesis Is Regulated by Plk4 and Is Essential for Zygote Development in Drosophila

Centrosome reduction is the decrease in centrosomal components during spermatid differentiation (spermiogenesis). It is one of several dramatic subcellular reorganizations that lead to spermatozoa formation common to a wide range of animals. However, the mechanism underlying centrosome reduction is unknown and its functions are unclear. Here, we show that in Drosophila melanogaster spermiogenesis, the quantity of centrosomal proteins is dramatically reduced; for example, Asterless (Asl) is reduced ∼500-fold and is barely detected in spermatozoa. Asl reduction is regulated through a subset of its domains by the master regulator of centriole duplication Plk4 and by the ubiquitin ligase that targets Plk4 for degradation: Slimb. When Asl reduction is attenuated by Asl overexpression, plk4 mutations, Plk4 RNAi, or Slimb overexpression, Asl levels are higher in spermatozoa, resulting in embryos with reduced viability. Significantly, overexpressing Plk4 and Asl simultaneously, or combining plk4 and slimb mutations, balances their opposing effects on Asl reduction, restoring seemingly normal fertility. This suggests that increased Asl levels cause the observed reduced fertility and not other pleotropic effects. Attenuation of Asl reduction also causes delayed development and a failure to form astral microtubules in the zygote. Together, we provide the first insight into a molecular mechanism that regulates centrosome reduction and the first direct evidence that centrosome reduction is essential for post-fertilization development.