An intracellular activation of Smoothened that is independent of Hedgehog stimulation in Drosophila

Implication of GPRASP2 in the Proliferation and Hair Cell-Forming of Cochlear Supporting Cells

G protein-coupled receptor-associated sorting protein 2 (GPRASP2) has been identified as the causative gene for X-linked recessive syndromic hearing loss (SHL) in our previous study. However, the role of GPRASP2 in auditory function remains unclear. The present study demonstrated that Gprasp2 overexpression in mouse organoids promoted the proliferation of supporting cells (SCs), which was mainly mediated by the Hedgehog signalling pathway. Meanwhile, GPRASP2 promoted hair cell (HC) formation from SCs via β-catenin signalling. In addition, GPRASP2 deficiency resulted in increased lysosomal degradation of SMO protein, leading to decreased expression of β-catenin and the Hedgehog pathway transcription factor GLI1. In neomycin-treated mouse cochlear explant, the smoothened agonist (SAG) recured the HC loss and further facilitated AAV-ie-Gprasp2 to promote the proliferation of SCs and formation of HCs. Our results suggested that GPRASP2 could be a potential candidate for gene therapy in the regeneration of HCs.

Protein phosphatase 2A regulates blood cell proliferation and differentiation in Drosophila larval lymph glands

Protein phosphatase 2A (PP2A), one of the most abundant protein phosphatases, has divergent functions in multiple types of cells. Its inactivation has been closely associated with leukemia diseases. However, the physiological function of PP2A for hematopoiesis has been poorly understood in organisms. Drosophila hematopoiesis parallels the vertebrate counterpart in developmental and functional features but involves a much simpler hematopoietic system. Here, utilizing the Drosophila major larval hematopoietic organ lymph gland, we studied the function of PP2A for hematopoiesis in vivo. By knocking down the expression of Pp2A-29B that encodes the scaffold subunit of the PP2A holoenzyme complex, we found that PP2A silencing in the differentiating hemocytes resulted in their excessive proliferation. Furthermore, this PP2A inhibition downregulated the expression of Smoothened (Smo), a crucial component in the Hedgehog pathway, and smo overexpression was able to rescue the phenotypes of PP2A depletion, indicating that Smo functions as a downstream effector of PP2A to restrict the hemocyte proliferation. PDGF/VEGF-receptor (Pvr) overexpression also restored the Smo expression and lymph gland morphology of PP2A silencing, suggesting a PP2A-Pvr-Smo axis to regulate lymph gland growth and hemocyte proliferation. Moreover, inhibiting PP2A activity in the blood progenitor cells promoted their differentiation, but which was independent with Smo. Together, our data suggested that PP2A plays a dual role in the Drosophila lymph gland by preserving the progenitor population and restraining the hemocyte proliferation, to properly regulate the hematopoietic process.

β-arrestins Are Scaffolding Proteins Required for Shh-Mediated Axon Guidance

During nervous system development, Sonic hedgehog (Shh) guides developing commissural axons toward the floor plate of the spinal cord. To guide axons, Shh binds to its receptor Boc and activates downstream effectors such as Smoothened (Smo) and Src family kinases (SFKs). SFK activation requires Smo activity and is also required for Shh-mediated axon guidance. Here we report that β-arrestin1 and β-arrestin2 (β-arrestins) serve as scaffolding proteins that link Smo and SFKs in Shh-mediated axon guidance. We found that β-arrestins are expressed in rat commissural neurons. We also found that Smo, β-arrestins, and SFKs form a tripartite complex, with the complex formation dependent on β-arrestins. β-arrestin knockdown blocked the Shh-mediated increase in Src phosphorylation, demonstrating that β-arrestins are required to activate Src kinase downstream of Shh. β-arrestin knockdown also led to the loss of Shh-mediated attraction of rat commissural axons in axon turning assays. Expression of two different dominant-negative β-arrestins, β-arrestin1 V53D which blocks the internalization of Smo and β-arrestin1 P91G-P121E which blocks its interaction with SFKs, also led to the loss of Shh-mediated attraction of commissural axons. In vivo, the expression of these dominant-negative β-arrestins caused defects in commissural axon guidance in the spinal cord of chick embryos of mixed sexes. Thus we show that β-arrestins are essential scaffolding proteins that connect Smo to SFKs and are required for Shh-mediated axon guidance.

Regulation of Smoothened Trafficking and Abundance in Hedgehog Signaling

The GPCR-family protein Smoothened (Smo) is essential for Hedgehog (Hh) signal transduction in both insects and vertebrates. The regulation of subcellular localization and abundance of Smo is a critical step in Hh signaling. Recent studies have demonstrated that Smo is subjected to ubiquitination mediated by multiple E3 ubiquitin ligases, leading to Smo endocytosis and subsequent degradation through the proteasome- and lysosome-mediated pathways in Drosophila. Ubiquitination of Smo also promotes its ciliary exit in mammalian cells. Hh inhibits Smo ubiquitination by blocking E3 ligase recruitment and promoting Smo deubiquitination through the ubiquitin-specific protease 8 (USP8) in Drosophila. Inhibition of Smo ubiquitination by Hh promotes Smo cell surface accumulation in Drosophila and ciliary accumulation in mammalian cells. Interestingly, Hh also induces sumoylation of Smo in both Drosophila and mammalian cells, which promotes Smo cell surface/ciliary accumulation. This review focuses on how ubiquitination and sumoylation regulate Smo intracellular trafficking and abundance and how these processes are regulated by Hh.

Mechanisms of Smoothened Regulation in Hedgehog Signaling

The seven-transmembrane protein, Smoothened (SMO), has shown to be critical for the hedgehog (HH) signal transduction on the cell membrane (and the cilium in vertebrates). SMO is subjected to multiple types of post-translational regulations, including phosphorylation, ubiquitination, and sumoylation, which alter SMO intracellular trafficking and cell surface accumulation. Recently, SMO is also shown to be regulated by small molecules, such as oxysterol, cholesterol, and phospholipid. The activity of SMO must be very well balanced by these different mechanisms in vivo because the malfunction of SMO will not only cause developmental defects in early stages, but also induce cancers in late stages. Here, we discuss the activation and inactivation of SMO by different mechanisms to better understand how SMO is regulated by the graded HH signaling activity that eventually governs distinct development outcomes.

Topology-driven protein-protein interaction network analysis detects genetic sub-networks regulating reproductive capacity

Understanding the genetic regulation of organ structure is a fundamental problem in developmental biology. Here, we use egg-producing structures of insect ovaries, called ovarioles, to deduce systems-level gene regulatory relationships from quantitative functional genetic analysis. We previously showed that Hippo signalling, a conserved regulator of animal organ size, regulates ovariole number in Drosophila melanogaster. To comprehensively determine how Hippo signalling interacts with other pathways in this regulation, we screened all known signalling pathway genes, and identified Hpo-dependent and Hpo-independent signalling requirements. Network analysis of known protein-protein interactions among screen results identified independent gene regulatory sub-networks regulating one or both of ovariole number and egg laying. These sub-networks predict involvement of previously uncharacterised genes with higher accuracy than the original candidate screen. This shows that network analysis combining functional genetic and large-scale interaction data can predict function of novel genes regulating development.

Hedgehog signaling promotes lipolysis in adipose tissue through directly regulating Bmm/ATGL lipase

Hedgehog (Hh) signaling has been shown to regulate multiple developmental processes, however, it is unclear how it regulates lipid metabolism. Here, we demonstrate that Hh signaling exhibits potent activity in Drosophila fat body, which is induced by both locally expressed and midgut-derived Hh proteins. Inactivation of Hh signaling increases, whereas activation of Hh signaling decreases lipid accumulation. The major lipase Brummer (Bmm) acts downstream of Smoothened (Smo) in Hh signaling to promote lipolysis, therefore, the breakdown of triacylglycerol (TAG). We identify a critical Ci binding site in bmm promoter that is responsible to mediate Bmm expression induced by Hh signaling. Genomic mutation of the Ci binding site significantly reduces the expression of Bmm and dramatically decreases the responsiveness to Ci overexpression. Together, our findings provide a model for lipolysis to be regulated by Hh signaling, raising the possibility for Hh signaling to be involved in lipid metabolic and/or lipid storage diseases.

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.

From intra- to extracellular vesicles: extracellular vesicles in developmental signalling

Signalling from cell-to-cell is fundamental for determining differentiation and patterning. This communication can occur between adjacent and distant cells. Extracellular vesicles (EVs) are membrane-based structures thought to facilitate the long-distance movement of signalling molecules. EVs have recently been found to allow the transport of two major developmental signalling pathways: Hedgehog and Wnt. These signalling molecules undergo crucial post-translational lipid modifications, which anchor them to membranes and impede their free release into the extracellular space. Preparation of these ligands in EVs involves intracellular vesicle sorting in an endocytosis-dependent recycling process before secretion. In the present review, we discuss the most recent advances with regard to EV involvement in developmental signalling at a distance. We focus on the role of the protein complexes involved in EV genesis, and provide a comprehensive perspective of the contribution of these complexes to intracellular vesicle sorting of developmental signals for their extracellular secretion, reception and transduction.