The embryos of animals develop in a controlled manner that ensures that their tissues and organs form properly and at the right time. These processes depend on molecules called morphogens that are distributed throughout the embryo in specific ways and that are dispersed via extensions that protrude from the surfaces of cells. These extensions, called cytonemes, transport the morphogens across the distances that separate cells and transfer these molecules to target cells via direct contact. However, it was not known how cytonemes navigate to their targets.

The fruit fly Drosophila is commonly used to investigate how animals develop organs and tissues. Previous studies have shown that the development of one of the fly’s organs – the air sac primordium –relies on morphogens transported by cytonemes.Now, Huang and Kornberg reveal that these cytonemes navigate to their targets by using the composition of the mesh-like framework – referred to as the extracellular matrix – that surrounds animal tissues as a guide. Further experiments showed that the extracellular matrix between the cells that produce the morphogens and the cells of the air sac primordium is roughly arranged into layers. These layers contain different molecules and the cytonemes navigate within specific layers.

These findings reinforce the idea that the extracellular space is organized and regulated, and show that the extracellular matrix is essential for developmental signaling. Future challenges include understanding how the layers of the extracellular matrix form and how information is encoded in these layers for the cytonemes to decipher as they navigate to their targets.

The language of development has a small vocabulary of signaling proteins that consists in part of Fibroblast growth factor (FGF) and Bone morphogenic proteins such as Drosophila Decapentaplegic (Dpp). This language may be used in most or all metazoan organs. Studies of Drosophila, chick, zebrafish, and cultured human cells show that the signaling proteins that regulate development are transported along actin-based filopodia (cytonemes) and exchange at synapses where the cells that produce them contact the cells that receive and respond to them (Roy et al., 2014 and reviewed in Kornberg and Roy, 2014; Pröls et al., 2016). The large distances between the source and receiving cells in some of these contexts (as much as 100 µm in the wing disc and 150 µm in the chick limb bud) highlights the question that this work investigates - how cytonemes extend to reach their targets.

The cytonemes we characterized were made by the ASP, a tracheal tube that develops in the third instar larva under the influence of Dpp and FGF that are produced in the wing disc (Roy et al., 2011; Sato and Kornberg, 2002). The cytonemes that mediate the exchange of these proteins contain the Dpp receptor Thickveins (Tkv) or the FGF receptor Breathless (Btl), extending from the basal surface of the ASP cells and synapsing with Dpp- or FGF-producing disc cells, respectively (Roy et al., 2014). The ASP lies underneath the basement membrane that envelops the wing disc (Guha et al., 2009), and although the space they traverse has not been analyzed, it presumably has characteristics of prototypical extracellular matrix (ECM, reviewed in Broadie et al., 2011). The number and distribution of ASP cytonemes depend on the production of Dpp and FGF in the disc and on their respective receptors in the ASP, but it is not known whether the cells between the producing and receiving cells (henceforth called 'intermediate cells') also contribute to cytoneme-mediated signaling. Possible candidates that might have roles in these intermediate cells that we tested include components of the planar cell polarity (PCP) system, heparan sulfate proteoglycans (HSPGs) and integrins.

PCP is an aspect of cell polarity that establishes a singular, shared bipolar orientation across an epithelial sheet (reviewed in Goodrich and Strutt, 2011). In the insect cuticle, it is responsible for the coordinated and consistent orientation of hairs and bristles. PCP is also manifested in the asymmetric subcellular localization of proteins such as Frizzled (Fz, a seven-pass transmembrane protein), Dishevelled (Dsh) and Diego (Dgo, cytosolic proteins) to one side and Van Gogh (Vang, a four-pass transmembrane protein) and Prickle (Pk, a cytosolic protein) to the other. Fz, Dsh, Dgo, Vang and Pk are constituents of the core PCP pathway in Drosophila, and all are required for planar orientation and polarization. Absent any one and the hairs and bristles lack normal polarity. In Drosophila genetic mosaics, cells to one side of PCP mutant cells also have abnormal planar polarity (Casal et al., 2002; Taylor et al., 1998; Vinson and Adler, 1987), leading to the idea that PCP involves a system of cell-cell interactions that coordinate the polarity of neighboring cells and propagate orientation long-range.

PCP components also have other roles. Studies of cells deficient for components of the PCP system in Drosophila (Djiane et al., 2005; Harumoto et al., 2010) and mouse (Tao et al., 2009; Vandenberg and Sassoon, 2009) report loss of several features of normal apical-basal polarity. PCP components have been implicated in the deposition and remodeling of the ECM (Dohn et al., 2013; Goto et al., 2005; Williams et al., 2012a, 2012b), axon guidance (Fenstermaker et al., 2010; Mrkusich et al., 2011; Shafer et al., 2011; Gombos et al., 2015 and reviewed in Yam and Charron, 2013) and cell migration (Carreira-Barbosa et al., 2003; Dohn et al., 2013; Roszko et al., 2015; Tatin et al., 2013).

The glypicans Dally and Dally-like (Dlp) are components of the ECM that contribute essential functions to signaling (Han et al., 2004; Fujise et al., 2003; Han et al., 2005; Lin and Perrimon, 1999; Yan and Lin, 2007; Yan et al., 2010). They are glycophosphatidylinositol (GPI)-anchored and are modified with heparan sulfate glycosaminoglycan (GAG) chains. Although an intracellular function has been reported for Dlp in Hh-producing cells (Callejo et al., 2011), it is assumed that both Dally and Dlp function externally. Models proposed for the roles of Dally and Dlp include binding as co-receptors of signaling proteins (Fujise et al., 2003; Kim et al., 2011; Lin and Perrimon, 1999) and participating in either a process of surface diffusion that involves repeated cycles of signaling protein-HSPG binding, dissociation, sliding and re-association with adjacent HSPG binding sites (Han et al., 2004; Schlessinger et al., 1995) or repeated cycles of ligand-HSPG binding, endocytic uptake, and transcytosis (Yan and Lin, 2009). Alternatively, the recent report that cytonemes do not pass over patches of cells that are deficient for GAG-modified HSPGs suggests that impairment of cytoneme-mediated transport may account for the mutant phenotypes (Bischoff et al., 2013).

Integrin function is required for axonal pathfinding, TGF-β signaling and for interactions of migrating cells and cell protrusions with the ECM (Dominguez-Giménez et al., 2007; Han et al., 2012; Myers et al., 2011; Robles and Gomez, 2006; Vuoriluoto et al., 2008 and reviewed in Munger and Sheppard, 2011). Although the basis for these roles has not been defined, we investigated whether integrins have an essential functional role in cytoneme-mediated signaling in the ASP system.

The ASP is a powerful system for studies of cell-cell signaling. Its large cells, easy accessibility for visual analysis, dependence on and sensitivity to paracrine signaling, and robust and abundant cytonemes offer many ways to exploit Drosophila’s genetic toolkit. We took advantage of these attributes to address the general question of targeting, asking how ASP cytonemes navigate across the wing disc in order to synapse specifically with cells that express either Dpp or FGF. We focused on the 'intermediate' disc cells that are situated between ASP cells that extend cytonemes and the disc cells they target, and we identified several functions that the intermediate cells must express in order for ASP cytonemes to mediate signaling. The results show that the extracellular environment in which cytonemes navigate is organized and structured and provides essential support that is specific to different cytoneme types.

Cytonemes are dynamic, and their orientations change during development as the tissues that make them grow and the spatial relationship between the cells that produce signaling proteins and the responding cells changes (Bischoff et al., 2013; Roy et al., 2011). They also change in response to ectopic over-expression of signaling proteins (Guha et al., 2009; Roy et al., 2011; Sato and Kornberg, 2002). These properties suggest that cytonemes are dependent on an active targeting mechanism that conceivably might involve: (1) a random search; (2) chemotaxis informed by an attractant; or (3) structural information encoded in their environment. Although the chemotaxis model seems unlikely because cytonemes that deliver signaling proteins orient to receiving cells (Bischoff et al., 2013; Callejo et al., 2011), and although the results we report do not rule out a role for random search, our results indicate that the extracellular environment created by intermediate cells is essential and raise the question how cytonemes interact with it.

We found that the PCP components pk and Vang are essential for ASP signaling and that ASP cytonemes did not extend over intermediate cells that lack pk or Vang function (Figure 3). The strong mutant phenotypes in the ASP had not been noted in previous studies of PCP mutants, the most likely reason being that the dorsal air sacs are not visible in intact flies and flies that lack dorsal air sacs are viable (Guha et al., 2009). Because components of the PCP system of the disc has been principally associated with epithelial planar polarity and with polarized subcellular distributions of its core components, the question arises what roles its components might have that affect cytonemes that extend in the extracellular environment.

In other contexts, some PCP components have been implicated in processes that may not be directly involved in epithelial planar polarity - in integrin signaling (Lewellyn et al., 2013), microtubule polarity (Ehaideb et al., 2014), axon guidance (Matsubara et al., 2011; Shimizu et al., 2011; reviewed in Yam and Charron 2013), polarized extensions of epithelial cells (Peng et al., 2012), neural tube closure (Kibar et al., 2001), directed cell movements (Tatin et al., 2013) and non-canonical Wnt signaling (reviewed in Karner et al., 2006). Various mechanisms have been proposed, including integration of cell-matrix and cell-cell interactions (Dohn et al., 2013), polarized assembly of fibronectin (Goto et al., 2005), and Mmp-dependent remodeling of the ECM (Williams et al., 2012). Polarized rows of laminin have been observed in the basement membrane at the basal surface of follicle cells in the Drosophila ovary (Gutzeit et al., 1991) that may involve receptor-mediated alignment of the cytoskeleton with the ECM (Frydman and Spradling, 2001). Although the functional link between the PCP system and these processes has not been established and although the possibility remains that the roles of the PCP components in these systems are unrelated to their roles in planar polarity, our working assumption is that axon pathfinding, polarized extensions, directed cell migrations and cytoneme navigation may share a requirement for features in the substrates they encounter that are dependent on cell polarity.

We discovered that pk and Vang mutant cells had reduced amounts of Dally, Dlp and laminin (Figure 5). This finding calls into question the extent to which the mutant phenotypes might be due to ECM defects and an indirect consequence of Pk or Vang mis-localization. The related findings that ECM collagen, laminin, and HSPGs are stratified in layers, and that the cytoneme subtypes that contain either Tkv or Btl navigate specifically in the Dally and Dlp ECM layers, respectively (Figures 6,7), indicates that the ECM is organized, partitioned and regulated to an unanticipated degree and that the ECM might be directly involved in the process of cytoneme extension and pathfinding.

Specific requirements of Dally for Dpp signaling (Dejima et al., 2011) and of Dlp for FGF signaling (Yan and Lin 2007) have been reported previously and have been interpreted as evidence that Dally and Dlp function as co-receptors; modifications of the Dally and Dlp heparan sulfate backbones have been proposed as structures that provide specificity for the respective signaling systems (Kamimura et al., 2006 and reviewed in Nakato and Kimata, 2002). Our observation that Tkv- and Btl-containing cytoneme subtypes were not present in areas that lacked Dally or Dlp, respectively (Figure 6), suggests that each HSPG provides a substrate that supports cytoneme extension and/or stability. This implies that cytonemes interact directly and specifically with the HSPGs and that the roles of these ECM components are not limited to co-receptor functions.

A generally held model posits that axons, migrating cells and cells in developmental fields interpret secreted guidance cues and signaling proteins that encode positional information in the form of chemogradient distributions that are stored in the ECM. HSPGs were proposed to function both to distribute the guidance cues and signaling proteins, and as co-receptors for signaling proteins (Fujise et al., 2003; Yan and Lin, 2007; Yan et al., 2010). Our discovery that cytonemes traffic signaling proteins between cells (Roy et al., 2014) revealed that signaling proteins that have been observed between signal producing and receiving cells are cytoneme-associated and are neither extracellular nor ECM-bound (Roy and Kornberg, 2015). Our findings are consistent with the idea that secreted signaling proteins and signaling protein receptors are not distributed in the extracellular environment and are not bound to the ECM, but rather that the cytonemes that mediate Dpp and FGF signaling contact the ECM directly in ways that involve both integrins and specific HSPG interactions.

1. 1. Adler PN
   2. Taylor J
   3. Charlton J

   (2000) The domineering non-autonomy of frizzled and van Gogh clones in the Drosophila wing is a consequence of a disruption in local signaling

   Mechanisms of Development 96:197–207.

   https://doi.org/10.1016/S0925-4773(00)00392-0

   • Google Scholar
2. 1. Akiyama T
   2. Kamimura K
   3. Firkus C
   4. Takeo S
   5. Shimmi O
   6. Nakato H

   (2008) Dally regulates Dpp morphogen gradient formation by stabilizing Dpp on the cell surface

   Developmental Biology 313:408–419.

   https://doi.org/10.1016/j.ydbio.2007.10.035

   • Google Scholar
3. 1. Bischoff M
   2. Gradilla AC
   3. Seijo I
   4. Andrés G
   5. Rodríguez-Navas C
   6. González-Méndez L
   7. Guerrero I

   (2013) Cytonemes are required for the establishment of a normal Hedgehog morphogen gradient in Drosophila epithelia

   Nature Cell Biology 15:1269–1281.

   https://doi.org/10.1038/ncb2856

   • Google Scholar
4. 1. Broadie K
   2. Baumgartner S
   3. Prokop A

   (2011) Extracellular matrix and its receptors in Drosophila neural development

   Developmental Neurobiology 71:1102–1130.

   https://doi.org/10.1002/dneu.20935

   • Google Scholar
5. 1. Callejo A
   2. Bilioni A
   3. Mollica E
   4. Gorfinkiel N
   5. Andrés G
   6. Ibáñez C
   7. Torroja C
   8. Doglio L
   9. Sierra J
   10. Guerrero I

   (2011) Dispatched mediates Hedgehog basolateral release to form the long-range morphogenetic gradient in the Drosophila wing disk epithelium

   PNAS 108:12591–12598.

   https://doi.org/10.1073/pnas.1106881108

   • Google Scholar
6. 1. Carreira-Barbosa F
   2. Concha ML
   3. Takeuchi M
   4. Ueno N
   5. Wilson SW
   6. Tada M

   (2003) Prickle 1 regulates cell movements during gastrulation and neuronal migration in zebrafish

   Development 130:4037–4046.

   https://doi.org/10.1242/dev.00567

   • Google Scholar
7. 1. Casal J
   2. Struhl G
   3. Lawrence PA

   (2002) Developmental compartments and planar polarity in Drosophila

   Current Biology 12:1189–1198.

   https://doi.org/10.1016/S0960-9822(02)00974-0

   • Google Scholar
8. 1. Dejima K
   2. Kanai MI
   3. Akiyama T
   4. Levings DC
   5. Nakato H

   (2011) Novel contact-dependent bone morphogenetic protein (BMP) signaling mediated by heparan sulfate proteoglycans

   Journal of Biological Chemistry 286:17103–17111.

   https://doi.org/10.1074/jbc.M110.208082

   • Google Scholar
9. 1. Djiane A
   2. Yogev S
   3. Mlodzik M

   (2005) The apical determinants aPKC and dPatj regulate Frizzled-dependent planar cell polarity in the Drosophila eye

   Cell 121:621–631.

   https://doi.org/10.1016/j.cell.2005.03.014

   • Google Scholar
10. 1. Dohn MR
    2. Mundell NA
    3. Sawyer LM
    4. Dunlap JA
    5. Jessen JR

    (2013) Planar cell polarity proteins differentially regulate extracellular matrix organization and assembly during zebrafish gastrulation

    Developmental Biology 383:39–51.

    https://doi.org/10.1016/j.ydbio.2013.08.027

    • Google Scholar
11. 1. Domínguez-Giménez P
    2. Brown NH
    3. Martín-Bermudo MD

    (2007) Integrin-ECM interactions regulate the changes in cell shape driving the morphogenesis of the Drosophila wing epithelium

    Journal of Cell Science 120:1061–1071.

    https://doi.org/10.1242/jcs.03404

    • Google Scholar
12. 1. Ehaideb SN
    2. Iyengar A
    3. Ueda A
    4. Iacobucci GJ
    5. Cranston C
    6. Bassuk AG
    7. Gubb D
    8. Axelrod JD
    9. Gunawardena S
    10. Wu CF
    11. Manak JR

    (2014) prickle modulates microtubule polarity and axonal transport to ameliorate seizures in flies

    PNAS 111:11187–11192.

    https://doi.org/10.1073/pnas.1403357111

    • Google Scholar
13. 1. Erkner A
    2. Roure A
    3. Charroux B
    4. Delaage M
    5. Holway N
    6. Coré N
    7. Vola C
    8. Angelats C
    9. Pagès F
    10. Fasano L
    11. Kerridge S

    (2002)

    Grunge, related to human Atrophin-like proteins, has multiple functions in Drosophila development

    Development 129:1119–1129.

    • Google Scholar
14. 1. Fanto M
    2. Clayton L
    3. Meredith J
    4. Hardiman K
    5. Charroux B
    6. Kerridge S
    7. McNeill H

    (2003) The tumor-suppressor and cell adhesion molecule Fat controls planar polarity via physical interactions with Atrophin, a transcriptional co-repressor

    Development 130:763–774.

    https://doi.org/10.1242/dev.00304

    • Google Scholar
15. 1. Feinberg EH
    2. Vanhoven MK
    3. Bendesky A
    4. Wang G
    5. Fetter RD
    6. Shen K
    7. Bargmann CI

    (2008) GFP Reconstitution Across Synaptic Partners (GRASP) defines cell contacts and synapses in living nervous systems

    Neuron 57:353–363.

    https://doi.org/10.1016/j.neuron.2007.11.030

    • Google Scholar
16. 1. Fenstermaker AG
    2. Prasad AA
    3. Bechara A
    4. Adolfs Y
    5. Tissir F
    6. Goffinet A
    7. Zou Y
    8. Pasterkamp RJ

    (2010) Wnt/planar cell polarity signaling controls the anterior-posterior organization of monoaminergic axons in the brainstem

    Journal of Neuroscience 30:16053–16064.

    https://doi.org/10.1523/JNEUROSCI.4508-10.2010

    • Google Scholar
17. 1. Fessler LI
    2. Campbell AG
    3. Duncan KG
    4. Fessler JH

    (1987) Drosophila laminin: characterization and localization

    The Journal of Cell Biology 105:2383–2391.

    https://doi.org/10.1083/jcb.105.5.2383

    • Google Scholar
18. 1. Frydman HM
    2. Spradling AC

    (2001)

    The receptor-like tyrosine phosphatase lar is required for epithelial planar polarity and for axis determination within drosophila ovarian follicles

    Development 128:3209–3220.

    • Google Scholar
19. 1. Fujise M
    2. Takeo S
    3. Kamimura K
    4. Matsuo T
    5. Aigaki T
    6. Izumi S
    7. Nakato H

    (2003) Dally regulates Dpp morphogen gradient formation in the Drosophila wing

    Development 130:1515–1522.

    https://doi.org/10.1242/dev.00379

    • Google Scholar
20. 1. Gombos R
    2. Migh E
    3. Antal O
    4. Mukherjee A
    5. Jenny A
    6. Mihály J

    (2015) The formin DAAM functions as molecular effector of the planar cell polarity pathway during axonal development in Drosophila

    Journal of Neuroscience 35:10154–10167.

    https://doi.org/10.1523/JNEUROSCI.3708-14.2015

    • Google Scholar
21. 1. Goodrich LV
    2. Strutt D

    (2011) Principles of planar polarity in animal development

    Development 138:1877–1892.

    https://doi.org/10.1242/dev.054080

    • Google Scholar
22. 1. Goto T
    2. Davidson L
    3. Asashima M
    4. Keller R

    (2005) Planar cell polarity genes regulate polarized extracellular matrix deposition during frog gastrulation

    Current Biology 15:787–793.

    https://doi.org/10.1016/j.cub.2005.03.040

    • Google Scholar
23. 1. Gotwals PJ
    2. Fessler LI
    3. Wehrli M
    4. Hynes RO

    (1994) Drosophila PS1 integrin is a laminin receptor and differs in ligand specificity from PS2

    PNAS 91:11447–11451.

    https://doi.org/10.1073/pnas.91.24.11447

    • Google Scholar
24. 1. Graner MW
    2. Bunch TA
    3. Baumgartner S
    4. Kerschen A
    5. Brower DL

    (1998) Splice variants of the Drosophila PS2 integrins differentially interact with RGD-containing fragments of the extracellular proteins tiggrin, ten-m, and D-laminin 2

    Journal of Biological Chemistry 273:18235–18241.

    https://doi.org/10.1074/jbc.273.29.18235

    • Google Scholar
25. 1. Guha A
    2. Lin L
    3. Kornberg TB

    (2009) Regulation of Drosophila matrix metalloprotease Mmp2 is essential for wing imaginal disc:trachea association and air sac tubulogenesis

    Developmental Biology 335:317–326.

    https://doi.org/10.1016/j.ydbio.2009.09.005

    • Google Scholar
26. 1. Gutzeit HO
    2. Eberhardt W
    3. Gratwohl E

    (1991)

    Laminin and basement membrane-associated microfilaments in wild-type and mutant Drosophila ovarian follicles

    Journal of Cell Science 100 (Pt 4):781–788.

    • Google Scholar
27. 1. Han C
    2. Belenkaya TY
    3. Wang B
    4. Lin X

    (2004) Drosophila glypicans control the cell-to-cell movement of Hedgehog by a dynamin-independent process

    Development 131:601–611.

    https://doi.org/10.1242/dev.00958

    • Google Scholar
28. 1. Han C
    2. Wang D
    3. Soba P
    4. Zhu S
    5. Lin X
    6. Jan LY
    7. Jan YN

    (2012) Integrins regulate repulsion-mediated dendritic patterning of drosophila sensory neurons by restricting dendrites in a 2D space

    Neuron 73:64–78.

    https://doi.org/10.1016/j.neuron.2011.10.036

    • Google Scholar
29. 1. Han C
    2. Yan D
    3. Belenkaya TY
    4. Lin X

    (2005) Drosophila glypicans Dally and Dally-like shape the extracellular Wingless morphogen gradient in the wing disc

    Development 132:667–679.

    https://doi.org/10.1242/dev.01636

    • Google Scholar
30. 1. Harumoto T
    2. Ito M
    3. Shimada Y
    4. Kobayashi TJ
    5. Ueda HR
    6. Lu B
    7. Uemura T

    (2010) Atypical cadherins Dachsous and Fat control dynamics of noncentrosomal microtubules in planar cell polarity

    Developmental Cell 19:389–401.

    https://doi.org/10.1016/j.devcel.2010.08.004

    • Google Scholar
31. 1. Huang H
    2. Kornberg TB

    (2015) Myoblast cytonemes mediate Wg signaling from the wing imaginal disc and delta-notch signaling to the air sac primordium

    eLife 4:e06114.

    https://doi.org/10.7554/eLife.06114

    • Google Scholar
32. 1. Kamimura K
    2. Koyama T
    3. Habuchi H
    4. Ueda R
    5. Masu M
    6. Kimata K
    7. Nakato H

    (2006) Specific and flexible roles of heparan sulfate modifications in Drosophila FGF signaling

    The Journal of Cell Biology 174:773–778.

    https://doi.org/10.1083/jcb.200603129

    • Google Scholar
33. 1. Karner C
    2. Wharton KA
    3. Carroll TJ

    (2006) Apical-basal polarity, Wnt signaling and vertebrate organogenesis

    Seminars in Cell & Developmental Biology 17:214–222.

    https://doi.org/10.1016/j.semcdb.2006.05.007

    • Google Scholar
34. 1. Kibar Z
    2. Vogan KJ
    3. Groulx N
    4. Justice MJ
    5. Underhill DA
    6. Gros P

    (2001) Ltap, a mammalian homolog of Drosophila Strabismus/Van Gogh, is altered in the mouse neural tube mutant Loop-tail

    Nature Genetics 28:251–255.

    https://doi.org/10.1038/90081

    • Google Scholar
35. 1. Kim MS
    2. Saunders AM
    3. Hamaoka BY
    4. Beachy PA
    5. Leahy DJ

    (2011) Structure of the protein core of the glypican Dally-like and localization of a region important for hedgehog signaling

    PNAS 108:13112–13117.

    https://doi.org/10.1073/pnas.1109877108

    • Google Scholar
36. 1. Kornberg TB
    2. Roy S

    (2014) Communicating by touch--neurons are not alone

    Trends in Cell Biology 24:370–376.

    https://doi.org/10.1016/j.tcb.2014.01.003

    • Google Scholar
37. 1. Lewellyn L
    2. Cetera M
    3. Horne-Badovinac S

    (2013) Misshapen decreases integrin levels to promote epithelial motility and planar polarity in Drosophila

    The Journal of Cell Biology 200:721–729.

    https://doi.org/10.1083/jcb.201209129

    • Google Scholar
38. 1. Lin X
    2. Perrimon N

    (1999) Dally cooperates with Drosophila Frizzled 2 to transduce Wingless signalling

    Nature 400:281–284.

    https://doi.org/10.1038/22343

    • Google Scholar
39. 1. Martin D
    2. Zusman S
    3. Li X
    4. Williams EL
    5. Khare N
    6. DaRocha S
    7. Chiquet-Ehrismann R
    8. Baumgartner S

    (1999) wing blister, a new Drosophila laminin alpha chain required for cell adhesion and migration during embryonic and imaginal development

    The Journal of Cell Biology 145:191–201.

    https://doi.org/10.1083/jcb.145.1.191

    • Google Scholar
40. 1. Matsubara D
    2. Horiuchi SY
    3. Shimono K
    4. Usui T
    5. Uemura T

    (2011) The seven-pass transmembrane cadherin Flamingo controls dendritic self-avoidance via its binding to a LIM domain protein, Espinas, in Drosophila sensory neurons

    Genes & Development 25:1982–1996.

    https://doi.org/10.1101/gad.16531611

    • Google Scholar
41. 1. Morillo Prado JR
    2. Chen X
    3. Fuller MT

    (2012) Polycomb group genes Psc and Su(z)2 maintain somatic stem cell identity and activity in Drosophila

    PLoS One 7:e52892.

    https://doi.org/10.1371/journal.pone.0052892

    • Google Scholar
42. 1. Mrkusich EM
    2. Flanagan DJ
    3. Whitington PM

    (2011) The core planar cell polarity gene prickle interacts with flamingo to promote sensory axon advance in the Drosophila embryo

    Developmental Biology 358:224–230.

    https://doi.org/10.1016/j.ydbio.2011.07.032

    • Google Scholar
43. 1. Munger JS
    2. Sheppard D

    (2011) Cross talk among TGF-β signaling pathways, integrins, and the extracellular matrix

    Cold Spring Harbor Perspectives in Biology 3:a005017.

    https://doi.org/10.1101/cshperspect.a005017

    • Google Scholar
44. 1. Myers JP
    2. Santiago-Medina M
    3. Gomez TM

    (2011) Regulation of axonal outgrowth and pathfinding by integrin-ECM interactions

    Developmental Neurobiology 71:901–923.

    https://doi.org/10.1002/dneu.20931

    • Google Scholar
45. 1. Nakato H
    2. Kimata K

    (2002) Heparan sulfate fine structure and specificity of proteoglycan functions

    Biochimica Et Biophysica Acta (BBA) - General Subjects 1573:312–318.

    https://doi.org/10.1016/S0304-4165(02)00398-7

    • Google Scholar
46. 1. Ninov N
    2. Menezes-Cabral S
    3. Prat-Rojo C
    4. Manjón C
    5. Weiss A
    6. Pyrowolakis G
    7. Affolter M
    8. Martín-Blanco E

    (2010) Dpp signaling directs cell motility and invasiveness during epithelial morphogenesis

    Current Biology 20:513–520.

    https://doi.org/10.1016/j.cub.2010.01.063

    • Google Scholar
47. 1. Peng Y
    2. Han C
    3. Axelrod JD

    (2012) Planar polarized protrusions break the symmetry of EGFR signaling during Drosophila bract cell fate induction

    Developmental Cell 23:507–518.

    https://doi.org/10.1016/j.devcel.2012.07.016

    • Google Scholar
48. 1. Persson U
    2. Izumi H
    3. Souchelnytskyi S
    4. Itoh S
    5. Grimsby S
    6. Engström U
    7. Heldin CH
    8. Funa K
    9. ten Dijke P

    (1998) The L45 loop in type I receptors for TGF-beta family members is a critical determinant in specifying Smad isoform activation

    FEBS Letters 434:83–87.

    https://doi.org/10.1016/S0014-5793(98)00954-5

    • Google Scholar
49. 1. Pröls F
    2. Sagar
    3. Scaal M

    (2016) Signaling filopodia in vertebrate embryonic development

    Cellular and Molecular Life Sciences 73:961–974.

    https://doi.org/10.1007/s00018-015-2097-6

    • Google Scholar
50. 1. Reich A
    2. Sapir A
    3. Shilo B

    (1999)

    Sprouty is a general inhibitor of receptor tyrosine kinase signaling

    Development 126:4139–4147.

    • Google Scholar
51. 1. Robles E
    2. Gomez TM

    (2006) Focal adhesion kinase signaling at sites of integrin-mediated adhesion controls axon pathfinding

    Nature Neuroscience 9:1274–1283.

    https://doi.org/10.1038/nn1762

    • Google Scholar
52. 1. Roszko I
    2. S Sepich D
    3. Jessen JR
    4. Chandrasekhar A
    5. Solnica-Krezel L

    (2015) A dynamic intracellular distribution of Vangl2 accompanies cell polarization during zebrafish gastrulation

    Development 142:2508–2520.

    https://doi.org/10.1242/dev.119032

    • Google Scholar
53. 1. Roy S
    2. Hsiung F
    3. Kornberg TB

    (2011) Specificity of Drosophila cytonemes for distinct signaling pathways

    Science 332:354–358.

    https://doi.org/10.1126/science.1198949

    • Google Scholar
54. 1. Roy S
    2. Huang H
    3. Liu S
    4. Kornberg TB

    (2014) Cytoneme-mediated contact-dependent transport of the Drosophila decapentaplegic signaling protein

    Science 343:1244624.

    https://doi.org/10.1126/science.1244624

    • Google Scholar
55. 1. Roy S
    2. Kornberg TB

    (2015) Paracrine signaling mediated at cell-cell contacts

    BioEssays 37:25–33.

    https://doi.org/10.1002/bies.201400122

    • Google Scholar
56. 1. Sato M
    2. Kornberg TB

    (2002) FGF is an essential mitogen and chemoattractant for the air sacs of the drosophila tracheal system

    Developmental Cell 3:195–207.

    https://doi.org/10.1016/S1534-5807(02)00202-2

    • Google Scholar
57. 1. Schlessinger J
    2. Lax I
    3. Lemmon M

    (1995) Regulation of growth factor activation by proteoglycans: what is the role of the low affinity receptors?

    Cell 83:357–360.

    https://doi.org/10.1016/0092-8674(95)90112-4

    • Google Scholar
58. 1. Schöck F
    2. Perrimon N

    (2003) Retraction of the Drosophila germ band requires cell-matrix interaction

    Genes & Development 17:597–602.

    https://doi.org/10.1101/gad.1068403

    • Google Scholar
59. 1. Shafer B
    2. Onishi K
    3. Lo C
    4. Colakoglu G
    5. Zou Y

    (2011) Vangl2 promotes Wnt/planar cell polarity-like signaling by antagonizing Dvl1-mediated feedback inhibition in growth cone guidance

    Developmental Cell 20:177–191.

    https://doi.org/10.1016/j.devcel.2011.01.002

    • Google Scholar
60. 1. Shimizu K
    2. Sato M
    3. Tabata T

    (2011) The Wnt5/planar cell polarity pathway regulates axonal development of the Drosophila mushroom body neuron

    Journal of Neuroscience 31:4944–4954.

    https://doi.org/10.1523/JNEUROSCI.0154-11.2011

    • Google Scholar
61. 1. Tao H
    2. Suzuki M
    3. Kiyonari H
    4. Abe T
    5. Sasaoka T
    6. Ueno N

    (2009) Mouse prickle1, the homolog of a PCP gene, is essential for epiblast apical-basal polarity

    PNAS 106:14426–14431.

    https://doi.org/10.1073/pnas.0901332106

    • Google Scholar
62. 1. Tatin F
    2. Taddei A
    3. Weston A
    4. Fuchs E
    5. Devenport D
    6. Tissir F
    7. Makinen T

    (2013) Planar cell polarity protein Celsr1 regulates endothelial adherens junctions and directed cell rearrangements during valve morphogenesis

    Developmental Cell 26:31–44.

    https://doi.org/10.1016/j.devcel.2013.05.015

    • Google Scholar
63. 1. Taylor J
    2. Abramova N
    3. Charlton J
    4. Adler PN

    (1998)

    Van Gogh: a new Drosophila tissue polarity gene

    Genetics 150:199–210.

    • Google Scholar
64. 1. Tree DR
    2. Shulman JM
    3. Rousset R
    4. Scott MP
    5. Gubb D
    6. Axelrod JD

    (2002) Prickle mediates feedback amplification to generate asymmetric planar cell polarity signaling

    Cell 109:371–381.

    https://doi.org/10.1016/S0092-8674(02)00715-8

    • Google Scholar
65. 1. Vandenberg AL
    2. Sassoon DA

    (2009) Non-canonical Wnt signaling regulates cell polarity in female reproductive tract development via van gogh-like 2

    Development 136:1559–1570.

    https://doi.org/10.1242/dev.034066

    • Google Scholar
66. 1. Vinson CR
    2. Adler PN

    (1987) Directional non-cell autonomy and the transmission of polarity information by the frizzled gene of Drosophila

    Nature 329:549–551.

    https://doi.org/10.1038/329549a0

    • Google Scholar
67. 1. Vuoriluoto K
    2. Jokinen J
    3. Kallio K
    4. Salmivirta M
    5. Heino J
    6. Ivaska J

    (2008) Syndecan-1 supports integrin alpha2beta1-mediated adhesion to collagen

    Experimental Cell Research 314:3369–3381.

    https://doi.org/10.1016/j.yexcr.2008.07.005

    • Google Scholar
68. 1. Williams BB
    2. Cantrell VA
    3. Mundell NA
    4. Bennett AC
    5. Quick RE
    6. Jessen JR

    (2012a) VANGL2 regulates membrane trafficking of MMP14 to control cell polarity and migration

    Journal of Cell Science 125:2141–2147.

    https://doi.org/10.1242/jcs.097964

    • Google Scholar
69. 1. Williams BB
    2. Mundell N
    3. Dunlap J
    4. Jessen J

    (2012b) The planar cell polarity protein VANGL2 coordinates remodeling of the extracellular matrix

    Communicative & Integrative Biology 5:325–328.

    https://doi.org/10.4161/cib.20291

    • Google Scholar
70. 1. Wu J
    2. Mlodzik M

    (2008) The frizzled extracellular domain is a ligand for Van Gogh/Stbm during nonautonomous planar cell polarity signaling

    Developmental Cell 15:462–469.

    https://doi.org/10.1016/j.devcel.2008.08.004

    • Google Scholar
71. 1. Yagi R
    2. Mayer F
    3. Basler K

    (2010) Refined LexA transactivators and their use in combination with the Drosophila Gal4 system

    PNAS 107:16166–16171.

    https://doi.org/10.1073/pnas.1005957107

    • Google Scholar
72. 1. Yam PT
    2. Charron F

    (2013) Signaling mechanisms of non-conventional axon guidance cues: the Shh, BMP and Wnt morphogens

    Current Opinion in Neurobiology 23:965–973.

    https://doi.org/10.1016/j.conb.2013.09.002

    • Google Scholar
73. 1. Yan D
    2. Lin X

    (2007) Drosophila glypican Dally-like acts in FGF-receiving cells to modulate FGF signaling during tracheal morphogenesis

    Developmental Biology 312:203–216.

    https://doi.org/10.1016/j.ydbio.2007.09.015

    • Google Scholar
74. 1. Yan D
    2. Lin X

    (2009) Shaping morphogen gradients by proteoglycans

    Cold Spring Harbor Perspectives in Biology 1:a002493.

    https://doi.org/10.1101/cshperspect.a002493

    • Google Scholar
75. 1. Yan D
    2. Wu Y
    3. Yang Y
    4. Belenkaya TY
    5. Tang X
    6. Lin X

    (2010) The cell-surface proteins Dally-like and Ihog differentially regulate Hedgehog signaling strength and range during development

    Development 137:2033–2044.

    https://doi.org/10.1242/dev.045740

    • Google Scholar
76. 1. Zervas CG
    2. Gregory SL
    3. Brown NH

    (2001) Drosophila integrin-linked kinase is required at sites of integrin adhesion to link the cytoskeleton to the plasma membrane

    The Journal of Cell Biology 152:1007–1018.

    https://doi.org/10.1083/jcb.152.5.1007

    • Google Scholar
77. 1. Zhang S
    2. Xu L
    3. Lee J
    4. Xu T

    (2002) Drosophila atrophin homolog functions as a transcriptional corepressor in multiple developmental processes

    Cell 108:45–56.

    https://doi.org/10.1016/S0092-8674(01)00630-4

    • Google Scholar

Author details

1. Hai Huang

   Cardiovascular Research Institute, University of California, San Francisco, San Francisco, United States

   Contribution

   HH, Conception and design, Acquisition of data, Analysis and interpretation of data, Drafting or revising the article

   Competing interests

   The authors declare that no competing interests exist.

2. Thomas B Kornberg

   Cardiovascular Research Institute, University of California, San Francisco, San Francisco, United States

   Contribution

   TBK, Conception and design, Analysis and interpretation of data

   For correspondence

   tkornberg@ucsf.edu

   Competing interests

   The authors declare that no competing interests exist.

   "This ORCID iD identifies the author of this article:" 0000-0002-6879-7066

• 2,197

  views

• 651

  downloads

• 42

  citations

Views, downloads and citations are aggregated across all versions of this paper published by eLife.