Hippo signaling regulates growth in Drosophila and mice. Loss of function of Hippo (Hpo) or of Warts (Wts), two kinases that lie at the center of the Hippo pathway, results in dramatic overgrowth of imaginal discs and of corresponding adult structures. The hpo gene was thus named after its mutant adult head phenotype, which resembles the hide of the hippopotamus. In imaginal discs, the Hippo pathway primarily affects the number of cells produced and has only minor effects on tissue patterning. Thus, the Hippo pathway is a key regulator of organ growth and tissue size in Drosophila (Halder, 2011).

Halder, G. and Johnson, R. L. (2011). Hippo signaling: growth control and beyond. Development 138: 9-22. PubMed ID: 21138973

Hippo signaling regulates Drosophila intestine stem cell proliferation through multiple pathways Hpo signaling regulates intestinal stem cell (ISC) proliferation through both cell-autonomous and non–cell-autonomous mechanisms. Hpo/Wts restricts the activity of Yki in the precursor cells to inhibit ISC proliferation. This cell-autonomous mechanism could be regulated by contact between ISC and basement membrane (BM), which is disrupted by tissue-damaging reagent dextran sulfate sodium (DSS). Hpo signaling also acts in the absorptive enterocytes (ECs) to restrict the production of ligands for the JAK-STAT and EGFR pathways, thereby inhibiting ISC proliferation by limiting the activities of these two pathways. Bleomycin and possibly bacterial infection (PE) cause damage of ECs and induce ISC proliferation through Yki-dependent and Yki-independent mechanisms (Ren, 2010).

Ren, F., Wang, B., Yue, T., Yun, E. Y., Ip, Y. T. and Jiang, J. (2010).Hippo signaling regulates Drosophila intestine stem cell proliferation through multiple pathways. Proc Natl Acad Sci U S A 107: 21064-21069. PubMed ID: 21078993

Hox cluster organization in Drosophila and mouse. The Drosophila Hox genes (top) are grouped into two genomic clusters: the Antennapedia (ANT-C) and Bithorax clusters (BX-C). Expression domains of the individual Hox genes within the ANT-C and BX-C along the anteroposterior (AP) axis of the fruitfly embryo match the array of the genes along the chromosome, displaying a property termed collinearity. Mice also possess a set of Hox genes similar to those found in Drosophila and their organisation along the chromosome as well as their order of expression along the AP axis also displayed collinearity. An important difference between these two systems is that the mouse has four clusters instead of one (Mallo, 2013).

Mallo, M. and Alonso, C. R. (2013). The regulation of Hox gene expression during animal development. Development 140: 3951-3963. PubMed ID: 24046316

Immune recognition of microbial agents in Drosophila: Toll pathway and immune deficiency (Imd) pathway. The Toll and The immune deficiency (Imd) pathways control inducible immune responses to bacteria and fungi in Drosophila through systemic production of antimicrobial peptides (AMPs). In the Toll pathway, immune recognition activates a proteolytic cascade that culminates in the maturation of the cytokine Spätzle, ultimately leading to the nuclear translocation of the nuclear factor-κB (NF-κB) transcription factor Dif, to induce the expression of AMP genes such as Drosomycin. Activation of the Imd pathway leads to the nuclear translocation of the NF-κB transcription factor Relish to activate the expression of AMP genes such as Diptericin (see Buchon, 2014).

Buchon, N., Silverman, N. and Cherry, S. (2014). Immunity in Drosophila melanogaster--from microbial recognition to whole-organism physiology. Nat Rev Immunol 14: 796-810. PubMed ID: 25421701

Nucleic acid recognition and antiviral defences in Drosophila. RNA viruses often encode structured RNAs or produce double-stranded RNA (dsRNA) intermediates. These are recognized and cleaved by Dicer-2 to form virus-derived small interfering RNAs (siRNAs), which are silenced through the RNA-induced silencing complex (RISC). The PIWI-interacting RNA (piRNA) pathway protects the cell from endogenous mobile genetic elements, especially those in the germ line. Some viruses can be directly sensed by Toll-7 to induce antiviral autophagy dependent on the conserved AKT pathway (see Buchon, 2014).

Buchon, N., Silverman, N. and Cherry, S. (2014). Immunity in Drosophila melanogaster--from microbial recognition to whole-organism physiology. Nat Rev Immunol 14: 796-810. PubMed ID: 25421701

Reactive oxygen species (ROS) is another core component of the immune response in the fly intestine. ROS response is basally activated by the gut microbiota and ingested microorganisms, and strongly induced by microbial infection. Microbially derived uracil triggers the adaptor guanine-nucleotide-binding protein q subunit-α (Gαq) and phospholipase Cβ (PLCβ) to induce the synthesis of inositol-3-phosphate, which in turn mediates the release of intracellular calcium and the transcription of the oxidase-encoding gene Duox (see Buchon, 2013).

Buchon, N., Broderick, N.A. and Lemaitre, B. (2013). Gut homeostasis in a microbial world: insights from Drosophila melanogaster. Nat Rev Microbiol 11: 615-626. PubMed ID: 23893105

Feedback regulation of the insulin signaling pathway by dFOXO in Drosophila. A. The insulin receptor inactivates dFOXO through dPI3K/dAkt. Activation of d4EBP may explain growth inhibition by dFOXO, whereas activation of dInR may provide a novel transcriptionally induced feedback control mechanism for the pathway. B. When nutrients are abundant, elevated levels of DILPs are secreted to activate the dInR pathway, and the resulting downstream signaling promotes growth, in part by inhibiting dFOXO. These favorable nutrient conditions would allow growth and development. However, when nutrients are limiting, DILPs are secreted at a reduced rate, and the dInR pathway is not activated (see Puig, 2003).

Puig, O., Marr, M.T., Ruhf, M.L. and Tjian, R. (2003). Control of cell number by Drosophila FOXO: downstream and feedback regulation of the insulin receptor pathway. Genes Dev 17: 2006-2020. PubMed ID: 12893776

Endocrine interactions regulating metabolic and proliferative homeostasis in Drosophila. Tissue systems and endocrine signals mediate specific responses to dietary changes and stress to maintain homeostasis in the adult animal. Drosophila insulin-like peptides (Dilps) coordinate multiple metabolic and regenerative responses to nutritional and stress conditions (see Wang, 2014).

Wang, L., Karpac, J. and Jasper, H. (2014). Promoting longevity by maintaining metabolic and proliferative homeostasis. J Exp Biol 217: 109-118. PubMed ID: 24353210

The IIS/TOR signaling pathway in fat body and the central nervous system of Drosophila. (A) Three of the eight insulin-like proteins (ILP) are expressed in a set of neurosecretory cells in the central nervous system (CNS). These ILPs activate insulin signaling in fat body cells. (B) In the CNS two ILPs are secreted by surface glia and activate the insulin signaling pathway in the neuroblasts to regulate the growth of this tissue. The anaplastic lymphoma kinase (Alk) and its ligand, Jelly belly (Jeb) promote growth of neuroblasts in starved larvae (Koyama, 2013).

Koyama, T., Mendes, C. C. and Mirth, C. K. (2013). Mechanisms regulating nutrition-dependent developmental plasticity through organ-specific effects in insects. Front Physiol 4: 263. PubMed ID: 24133450

Epithelial cells contacts hemolymph basally and secretes cuticle apically. Tyrosine taken up from the hemolymph is converted in epithelial cells to L-dopa by tyrosine hydroxylase (Pale) and then to dopamine by dopa decarboxylase. Some dopamine is released into the cuticle, where it is oxidized by Laccase 2 to dopamine quinone. In the presence of Yellow, dopamine quinone polymerizes to form black melanin. Some dopamine is conjugated with β-alanine by Ebony to produce N-β-alanyl dopamine, and the reverse reaction is catalysed by Tan. Upon release of N-β-alanyl dopamine into the cuticle, it is oxidized by Laccase 2 to a quinone, which mediates cuticle protein cross-linking (sclerotization) (Riedel, 2011).

Riedel, F., Vorkel, D. and Eaton, S. (2011). Megalin-dependent yellow endocytosis restricts melanization in the Drosophila cuticle. Development 138: 149-158. PubMed ID: 21138977

The fusion of myoblasts into multinucleate syncytia plays a fundamental role in muscle function, as it supports the formation of extended sarcomeric arrays, or myofibrils, within a large volume of cytoplasm. In myoblast fusion involves two cell types; founder cells/myotubes and fusion-competent myoblasts of Drosophila embryos. The represented proteins include components of the Rac1, Scar and WASp pathways and their regulators and cell-adhesion molecules. A fusion-competent myoblast migrates or extends filopodia to contact a founder cell or, in subsequent rounds of fusion, a syncytial myotube. Cell-surface adhesion molecules mediate recognition and adhesion between cells. Actin accumulates in the FCM, forming a large F-actin-based protrusion that pushes into the founder cell. A thin sheath of actin is present in the founder cell. One, or more, fusion pores form to allow mixing of cytoplasmic contents. The FCM is absorbed into the myotube, and the resulting syncytium continues additional rounds of fusion as needed (Abmayr, 2012).

Abmayr, S. M. and Pavlath, G. K. (2012). Myoblast fusion: lessons from flies and mice. Development 139: 641-656. PubMed ID: 22274696

Neuroblast temporal transitions in the embryonic CNS. Sequential transitions in neuroblast gene expression generate layered sublineage expression domains. During each temporal gene expression window, asymmetric NB divisions give rise to GMCs that are marked by the continued presence of the temporal factor that is expressed in the NB during its birth. These transcription factors are also detected in nascent postmitotic neurons and glia. Cells that express Hunchback (Hb) are positioned on the inner basal surface of the developing ganglion, and are pushed deeper into the developing neuromere upon the birth of subsequent lineages marked by expression of Kruppel, Pdm-1 (Nubbin) and Pdm-2, Castor, and Grainyhead (Brody, 2002).

Brody, T. and Odenwald, W. F. (2002). Cellular diversity in the developing nervous system: a temporal view from Drosophila. Development 129: 3763-3770. PubMed ID: 12135915

The ‘neuroblast clock’: a series of transcription factors that regulate neuroblast temporal identity. (A) Embryonic neuroblasts (NBs) consecutively express Hunchback (Hb), Seven up (Svp), Kruppel (Kr), Pdm1/2 (Pdm) and Castor (Cas); these are inherited by the ganglion mother cell (GMC). At the end of the embryonic stages, Castor-positive NBs enter quiescence. When division resumes in the larval stages, ventral nerve cord thoracic NBs transition from Castor to Svp expression (B) Larvae mutant for svp (svp−/− clones induced) do not switch off Castor expression and do not exit the cell cycle at the appropriate time the during pupal stages (see Homem, 2012).

Homem, C.C. and Knoblich, J.A. (2012).  Drosophila neuroblasts: a model for stem cell biology. Development 139: 4297-4310. PubMed ID: 23132240

Neurogenesis in the Optic Lobe: Neuroblast transitions. Medulla neurogenesis in the optic lobe depends on the sequential conversion of neuroepithelial (NE) cells to NBs. (A) Outer proliferation center (OPC) NE cells gradually convert into medulla NBs in a medial to lateral orientation. The advancement of the proneural wave is defined by Lethal of scute (L’sc) expression. (B) The gradual conversion of NE cells to NBs involves a switch from symmetric to asymmetric cell divisions. (C) The progression of the proneural wave from medial to lateral is negatively regulated by the Notch (N) pathway and positively regulated by the epidermal growth factor receptor (EGFR) pathway (Apitz, 2014).

Apitz, H. and Salecker, I. (2014). A challenge of numbers and diversity: neurogenesis in the Drosophila optic lobe. J Neurogenet 28: 233-249. PubMed ID: 24912777

Neurogenesis in the Optic Lobe: Transitions in Gene Expression. Temporal patterning of medulla NBs and Notch (N) signaling contribute to the generation of diverse neuron subtypes. (A) NE cells gradually convert into NBs. As medulla NBs age, they sequentially express the transcription factors Homothorax (Hth), Eyeless (Ey), Sloppy paired 1 and 2 (Slp), Dichaete (D), and Tailless (Tll). Each NB produces a column of medulla neurons. Progeny maintain the expression of the determinant present in the NB at the time of their birth. (B) Ey, Slp, and D are required for the transition to the next determinant. Slp, D, and Tll are necessary to repress the preceding factor in the series. Tll is sufficient but not required to repress D. (C) N-mediated binary cell fate choices further diversify lineages (Apitz, 2014).

Apitz, H. and Salecker, I. (2014). A challenge of numbers and diversity: neurogenesis in the Drosophila optic lobe. J Neurogenet 28: 233-249. PubMed ID: 24912777

Asymmetric cell division of Drosophila neuroblasts (NB) and sensory organ precursor (SOP) cells. (a) After delamination from the layer of the epithelium, the NB divides asymmetrically to generate a new NB and a ganglion mother cell (GMC). This process is guided and marked by apico-basal polarity. (b) Asymmetric division of the sensory organ precursor (SOP) cell of the Drosophila PNS, which gives rise to an anterior cell, pIIb and a posterior cell, pIIa. Aurora A and Numb are the main regulators of this division (Noatynska, 2013).

Noatynska, A., Tavernier, N., Gotta, M. and Pintard, L. (2013). Coordinating cell polarity and cell cycle progression: what can we learn from flies and worms? Open Biol 3: 130083. PubMed ID: 23926048

Nitric oxide signaling during Drosophila metamorphosis. In the prothoracic gland, nitric oxide synthase (NOS)-mediated production of NO prevents E75 function as a suppressor of DHR3, allowing DHR3 to induce βFTZ-F1 expression. βFTZ-F1 in turn promotes ecdysteroidogenesis. In peripheral tissues, E75 degradation leads to liberation of accumulated DHR3, allowing it to induce βFTZ-F1 expression  (see Venables, 2012).

Yamanaka, N. and O'Connor, M.B. (2011). Nitric oxide directly regulates gene expression during Drosophila development: need some gas to drive into metamorphosis? Genes Dev 25: 1459-1463. PubMed ID: 21764850

a. Notch signaling in Drosophila. Binding of the ligands, Delta or Serrate, to the Notch receptor leads to a series of specific cleavages of the Notch protein, and subsequently the liberation of its cytoplasmic domain (ICN). Suppressor of Hairless (Su(H)) binds to the ICN and this complex translocates to the nucleus where it activates the expression of target genes. b. In the wing imaginal disc of the third larval instar, Notch signalling is induced in two rows of cells that flank the presumptive wing margin. This causes expression of the secreted WNT-related wingless gene, which organizes gene expression in the vicinity of the wing margin. c. Segmentation of the vertebrate somitic mesoderm depends on an oscillating pattern of Notch and wingless related (Wnt)/Fibroblast growth factor (FGF) signalling that spreads along the anterior-posterior axis after being initiated in posterior-most cells (see Bier, 2005).

Bier, E. (2005). Drosophila, the golden bug, emerges as a tool for human genetics. Nat Rev Genet 6: 9-23. PubMed ID: 15630418

The Drosophila melanogaster primary PIWI-interacting RNA (piRNA) processing pathway. The primary antisense transcripts transcribed from transposons and/or the piRNA clusters are processed to piRNAs by unknown mechanisms and are loaded onto Aubergine or PIWI. piRNAs derived from the flamingo (flam) locus are exclusively loaded onto PIWI because flam is active only in ovarian somas. piRNA-induced silencing complexes produced through this mechanism act as a 'trigger' of the amplification loop (Siomi, 2011).

Siomi, M.C., Sato, K., Pezic, D. and Aravin, A.A. (2011). PIWI-interacting small RNAs: the vanguard of genome defence. Nat Rev Mol Cell Biol 12: 246-258. PubMed ID: 21427766

(A) At the postsynaptic membrane, shown at the bottom, Discs large (Dlg) localizes to spectrin-actin complexes. Homophilic adhesion between Fasciclin 2 (Fas2) transmembrane proteins links the presynaptic and postsynaptic sides, with the intracellular C-terminal domains anchored to the first and second PDZ domains of Dlg. The adducin Hu-li tai shao (Hts) is in a complex with Dlg at the postsynaptic membrane, though the interaction may not be direct. Hts also binds to the lipid Phosphatidylinositol 4,5-bisphosphate (PIP2) via the MARCKS-homology domain. (B) Hts promotes the accumulation of par-1 and camkII transcripts in the muscle cytoplasm through an as of yet identified mechanism. PAR-1 and CaMKII phosphorylate Dlg. Phosphorylation disrupts Dlg postsynaptic targeting. (C) Phosphorylation translocates Hts away from the postsynaptic membrane and hinders Hts' ability to regulate Dlg localization, presumably through the control of PAR-1 and CaMKII at the transcriptional level. Phosphorylation of the MARCKS-homology domain also inhibits Hts' ability to bind to PIP2 (Wang, 2014).

Neto engages the iGluR complexes extrajunctionally and together they traffic and cluster at the synapses, opposite from the active zones marked by T-bars. Neto and the essential iGluR subunits are limiting for formation of functional iGluR complexes at the NMJ and for growth of synaptic structures (Kim, 2014).

Kim, Y. J. and Serpe, M. (2013). Building a synapse: a complex matter. Fly (Austin) 7: 146-152. PubMed ID: 23680998

Wang, S. J., Tsai, A., Wang, M., Yoo, S., Kim, H. Y., Yoo, B., Chui, V., Kisiel, M., Stewart, B., Parkhouse, W., Harden, N. and Krieger, C. (2014). Phospho-regulated Drosophila adducin is a determinant of synaptic plasticity in a complex with Dlg and PIP2 at the larval neuromuscular junction. Biol Open 3: 1196-1206. PubMed ID: 25416060

Five evolutionarily conserved proteins - RIM (Drosophila Rab3 interacting molecule) , Munc13 (Drosophila unc-13), RIM-BP (Drosophila Rim-binding protein), &alpha-liprin (Drosophila Liprin-α), and ELKS (Drosophila Bruchpilot) proteins - form the core of active zones. SYD-1 is a Rho GAP that is essential for synapse assembly in invertebrates but its functional homolog is unknown in vertebrates. RIM, Munc13, and RIM-BP are multidomain proteins composed of a string of identifiable modules, whereas α-liprin and ELKS exhibit a simpler structure. The five core active zone proteins form a single large protein complex that docks and primes synaptic vesicles, recruits Ca²⁺ channels to the docked and primed vesicles, tethers the vesicles and Ca²⁺ channels to synaptic cell-adhesion molecules, and mediates synaptic plasticity (Südhof, 2013). Imaging of developing Drosophila glutamatergic synapses revealed that the Unc13B isoform was recruited to nascent active zones by the scaffolding proteins Syd-1 and Liprin-α, and Unc13A is positioned by Bruchpilot and Rim-binding protein complexes at maturing active zones (Bohme, 2016).

Sudhof, T. C. (2012). The presynaptic active zone. Neuron 75: 11-25. PubMed ID: 22794257

Bohme, M. A., et al. (2016). Active zone scaffolds differentially accumulate Unc13 isoforms to tune Ca2+ channel-vesicle coupling. Nat Neurosci [Epub ahead of print]. PubMed ID: 27526206

Schematic diagram of the reward circuits in the mushroom body (MB). Sugar ingestion activates multiple modulatory pathways, such as octopamine and Allatostatin A (AstA), that bidirectionally regulate distinct dopamine neurons. These dopamine neurons convey reward signals to the MB by their activation or inhibition and induce appetitive memory. A specific sub-class of these dopaminergic protocerebral anterior medial (PAM) neurons, called PAM-γ3, mediates both aversive and appetitive reinforcement through activation and suppression of their activity, respectively. Notably, transient inactivation of the basal activity of PAM-γ3 neurons substitutes for reward and induces appetitive memory formation. Interestingly, AstA, a neuropeptide that signals satiety, conveys inhibitory input onto PAM-γ3 neurons. These results highlight the bidirectional activity of defined dopaminergic neurons, which underlies encoding of behaviorally relevant appetitive and aversive values.

Yamagata, N., Hiroi, M., Kondo, S., Abe, A. and Tanimoto, H. (2016). Suppression of Dopamine Neurons Mediates Reward. PLoS Biol 14(12): e1002586. PubMed ID: 27997541

In the generation of planar cell polarity (PCP) the E3 ubiquitin ligase complex Cullin1(Cul1)/SkpA/Supernumerary limbs(Slimb) regulates the stability of one of the peripheral membrane components, Prickle (Pk). Excess Pk disrupts PCP feedback and prevents asymmetry. Pk was found to participate in negative feedback by mediating internalization of PCP complexes containing the transmembrane components Van Gogh (Vang) and Flamingo (Fmi), and that internalization is activated by oppositely oriented complexes within clusters. Pk also participates in positive feedback through an unknown mechanism promoting clustering. these results therefore identify a molecular mechanism underlying generation of asymmetry in PCP signaling (Cho, 2015).

Cho, B., Pierre-Louis, G., Sagner, A., Eaton, S. and Axelrod, J. D. (2015).Clustering and negative feedback by endocytosis in planar cell polarity signaling is modulated by ubiquitinylation of prickle. PLoS Genet 11: e1005259. PubMed ID: 25996914

Small RNA silencing pathways in flies: There are three small RNA silencing pathways in flies: 1) small interfering RNA (siRNA made from double-stranded RNA precursors that are processed by Dicer-2 ), 2) microRNAs (miRNAs produced as non-coding RNA molecules that function in RNA silencing and post-transcriptional regulation of gene expression) and 3) Piwi-interacting RNAs (piRNAs linked to both epigenetic and post-transcriptional gene silencing of retrotransposons in germ line cells). dsRNA precursors generate siRNA duplexes containing guide and passenger strands that are loaded into Argonaute2. miRNAs are are cleaved by Drosha to yield short precursor miRNAs that are further processed by DCR-1. Once loaded into AGO1, the miRNA strand guides translational repression of target RNAs. Antisense piRNAs are are preferentially loaded into Piwi or Aubergine and probably direct cleavage of transposon mRNA or chromatin modification at transposon loci (Ghildiyal, 2009).

Ghildiyal, M. and Zamore, P.D. (2009). Small silencing RNAs: an expanding universe. Nat Rev Genet 10: 94-108. PubMed ID: 21764850

(A) Coincidence detector model for how mammalian mTORC1 integrates signals from nutrients and growth factors to regulate growth. The Rag GTPases promote the localization of mTORC1 to the lysosomal surface in response to nutrients, and, at the lysosome, the Rheb GTPase activates its kinase activity in response to insulin and energy levels (see Drosophila Tor, Rag A-B and Rag C-D, Tsc1, Rheb), V-ATPase and the GATOR2 component Missing oocyte. (B) Schematic showing components of the nutrient-sensing pathway upstream of mTORC1, including the many multiprotein complexes that regulate the Rag GTPases as well as the amino acid sensors Sestrin2, CASTOR1, and SLC38A9, and the SAM sensor SAMTOR (see Sabatini, 2017).

Sabatini, D. M. (2017). Twenty-five years of mTOR: Uncovering the link from nutrients to growth. Proc Natl Acad Sci U S A 114(45): 11818-11825. PubMed ID: 29078414

Comparison of Drosophila immune deficiency (imd), Toll, and mammalian TLR signaling pathways. The Imd pathway is activated by DAP-type PGN binding of the PGRP-LC dimer and ultimately leads to the phosphorylation and activation of Relish and AP-1, which activate the transcription of AMP and stress genes, respectively. The Toll pathway is activated by Spz leading to the nuclear translocation of Dif. Mammalian TLRs are activated by bacterial-, viral-, and self-derived products. TLR activation ultimately results in activation and nuclear translocation of NF-κB and AP-1 (see Valanne, 2011).

Valanne, S., Wang, J.H. and Rämet, M. (2011). The Drosophila Toll signaling pathway. J Immunol 186: 649-656. PubMed ID: 21209287

Canonical Wnt signaling pathway in Drosophila. Binding of Wingless (Wg) to Frizzled (Fz) leads to Axin degradation, thus allowing Armadillo (Arm) accumulation and its subsequent translocation to the nucleus where it binds Tcf, displaces the Groucho co-repressor, and this leads to target gene expression (see Bejsovec, 2013).

Bejsovec, A. (2013). Wingless/Wnt signaling in Drosophila: the pattern and the pathway. Mol Reprod Dev 80: 882-894. PubMed ID: 24038436

The role of negative feedback inhibitors in Wingless (Wg) signaling. Drosophila Wg pathway components include the receptor Frizzled 2 (Fz2), Dishevelled (Dsh), Armadillo (Arm) and Pangolin (dTcf). Wg signaling is controlled by a number of induced inhibitors including Naked (Nkd) and Wingful/Notum (Wf). Wg also regulates Nemo expression and Nemo in turn can antagonize Wg during wing patterning (Zeng, 2004).

Zeng, Y. A. and Verheyen, E. M. (2004). Nemo is an inducible antagonist of Wingless signaling during Drosophila wing development. Development 131: 2911-2920. PubMed ID: 15169756