International Journal of Molecular Sciences Review The Case of the Scribble Polarity Module in Asymmetric Neuroblast Division in Development and Tumorigenesis Ana Carmena Instituto de Neurociencias, Consejo Superior de Investigaciones Científicas/Universidad Miguel Hernández, 03550 Sant Joan d’Alacant, Alicante, Spain; [email protected] Received: 20 March 2020; Accepted: 17 April 2020; Published: 20 April 2020 Abstract: The Scribble polarity module is composed by Scribble (Scrib), Discs large 1 (Dlg1) and Lethal (2) giant larvae (L(2)gl), a group of highly conserved neoplastic tumor suppressor genes (TSGs) from flies to humans. Even though the Scribble module has been profusely studied in epithelial cell polarity, the number of tissues and processes in which it is involved is increasingly growing. Here we discuss the role of the Scribble module in the asymmetric division of Drosophila neuroblasts (NBs), as well as the underlying mechanisms by which those TSGs act in this process. Finally, we also describe what we know about the consequences of mutating these genes in impairing the process of asymmetric NB division and promoting tumor-like overgrowth. Keywords: Scribble polarity module; asymmetric cell division; neuroblasts; tumorigenesis; Drosophila 1. The Scribble Polarity Module The Scribble polarity module is composed by Scribble (Scrib), Discs large 1 (Dlg1) and lethal (2) giant larvae (L(2)gl), all of which represent highly conserved neoplastic tumor suppressor genes (TSGs) from flies to humans [1–4]. l(2)gl alleles were first isolated in Drosophila by Bridges in 1930s [5], but their malignant mutant phenotype, an extensive overproliferation and tissue disorganization in imaginal epithelia and adult brain anlagen (optic lobe), was described years later in an spontaneous l(2)gl mutation (l(2)gl4)[6]. Thus, l(2)gl was the first example of a TSG, even before the term “tumor suppressor gene” came on stage [7–9], although the existence of a recessive class of cancer genes (“recessive oncogenes”) had already been suggested by Harris’ cell fusion experiments and Knudson’s retinoblastoma studies [10,11]. dlg1 was isolated few years later in a screen for mutant phenotypes in the imaginal discs that altered their morphology. dlg1 mutants showed hypertrophied and disorganized discs, a phenotype, as the authors pointed out, very similar to that shown by l(2)gl4 mutants (Stewart et al. 1972). The third component of the module, scrib, was identified twenty years ago in a screen for mutations that altered the embryonic epithelial architecture in Drosophila [12]. scrib phenotype was identical to that shown by l(2)gl and dlg1 mutants in embryonic, follicular and imaginal disc epithelia, strongly suggesting that all three genes acted in the same pathway to regulate cell growth and cell polarity [13]. A collaborative activity in a single pathway of scrib, dlg1 and l(2)gl was confirmed by epistatic and strong genetic interactions between them, as well as by their cell localization and activity interdependence [13]. This, along with other previous works showed that the three neoplastic TSGs of the Scribble module were also key regulators of apico-basal cell polarity [3,12–16]. In epithelia, mutants in all these neoplastic TSGs lead to massive overgrowth and tumor formation in a homozygote condition, i.e., when the whole epithelial tissue is mutant for the TSG. However, mutant clones for each of these TSGs, scrib, dlg1 and l(2)gl, do not overgrow, have poor survival, and are finally eliminated by cell competition, a process in which the surrounding wild-type cells activate a Int. J. Mol. Sci. 2020, 21, 2865; doi:10.3390/ijms21082865 www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2020, 21, x FOR PEER REVIEW 2 of 13 Int. J. Mol. Sci. 2020, 21, 2865 2 of 12 l(2)gl, do not overgrow, have poor survival, and are finally eliminated by cell competition, a process in which the surrounding wild-type cells activate a Jun N-terminal kinase (JNK)-mediated apoptosis Junin the N-terminal mutant cells kinase [17–22]. (JNK)-mediated An oncogenic, apoptosis activated in the form mutant of Ras, cells Ras [17V12–, 22can]. prevent An oncogenic, the cell activated death in formthese ofTSG Ras, singleRasV1 mutant2, can clones prevent by the transforming cell death inthe these pro-apoptotic TSG single fu mutantnction of clones JNK into by transforminga pro-growth theeffect, pro-apoptotic inducing the function formation of JNK of big into neoplastic a pro-growth tumoral effect, masses inducing [18,19,21,23]. the formation More recently, of big neoplastic though, tumoralit has been masses shown, [18, 19for,21 four,23]. different More recently, l(2)gl null though, mutant it has alleles, been shown,that l(2)gl for foursingle di mutantfferent l(2)gl clonesnull in mutanteye-antennal alleles, discs that behavel(2)gl single differently mutant from clones scrib/dlg1 in eye-antennal clones, in discs the sense behave that di fftheyerently are not from eliminatedscrib/dlg1 clones,by cell incompetition. the sense that In theyfact, arel(2)gl not clones eliminated overproliferate by cell competition. without affecting In fact, l(2)gl cell polarityclones overproliferate by repressing withoutthe Hippo aff ectingsignaling cell pathway polarity by[24,25] repressing. Intriguingly, the Hippo it has signaling been found pathway that not [24 ,only25]. Intriguingly,l(2)gl but also it dlg1 has beensingle found mutant that clones, not only in antennall(2)gl but discs, also overgrowdlg1 single by mutant activating clones, the inRas-MAPK antennal discs,signaling overgrow pathway, by activatingwhile scrib the clones Ras-MAPK do not signaling [26]. Thus, pathway, it might while be scribthat clonesthe consequences do not [26]. of Thus, losing it might these be TSGs that theare consequencescontext-dependent, of losing and thesethat the TSGs induced are context-dependent, levels of signaling andpathways, that the such induced as JNK levels or Ras-MAPK, of signaling in pathways,the mutant such clones as determine JNK or Ras-MAPK, the apoptosis in the or mutantsurvival clones of the determineclone. the apoptosis or survival of the clone.Scrib, Dlg1, and L(2)gl are all cytoplasmic, membrane-associated proteins that co-localize to the baso-lateralScrib, Dlg1, domain and L(2)glof epithelial are all cytoplasmic,cells. Scrib membrane-associatedand Dlg1 are multi proteinsPDZ (PSD-95/Dlg1/ZO-1) that co-localize to thedomain-containing baso-lateral domain proteins of epithelialthat act as cells.scaffolds, Scrib whereas and Dlg1 L(2)gl are is multia non-muscle PDZ (PSD-95 myosin/Dlg1 II binding/ZO-1) domain-containingprotein with WD-40 proteins repeats that and, act as unlike scaffolds, Scrib whereas and L(2)glDlg1, isis acortically non-muscle uniform myosin (Figure II binding 1) protein[1,3,12,22,27,28]. with WD-40 repeats and, unlike Scrib and Dlg1, is cortically uniform (Figure1)[ 1,3,12,22,27,28]. FigureFigure 1.1.The The Scribble Scribble polarity polarity module. module. The The modular modular structure structure of the of proteins the proteins belonging belonging to the Scribble to the module,Scribble Scrib,module, Dlg1, Scrib, and L(2)gl,Dlg1, isand shown. L(2)gl, The is LAP shown. (Leucine-rich The LAP repeats (Leucine-rich Aand PDZ repeats domains) Aand protein PDZ Scribdomains) and theprotein MAGUK Scrib (Membraneand the MAGUK Associated (Membran GUanylatee Associated Kinase) GUanylate protein Dlg1 Kinase) both containprotein PDZDlg1 domainsboth contain and additionalPDZ domains motifs, and such additional as 16 Leucine-Rich-Repeats motifs, such as 16 Leucine-Rich-Rep (LRRs, in Scrib) oreats a Src(LRRs, Homology in Scrib) 3 andor a aSrc GUuanylate Homology Kinase 3 and (SH3 a GUuanylate and GUK domains, Kinase (SH3 in Dlg1). and L(2)glGUK containsdomains, WD-40 in Dlg1). repeats. L(2)gl Conserved contains phosphorylationWD-40 repeats. Conserved sites are shown phosphorylation in both L(2)gl sites and are Scrib. shown in both L(2)gl and Scrib. SomeSome physicalphysical interactionsinteractions betweenbetween thethe ScribbleScribble modulemodule proteinsproteins havehave beenbeen describeddescribed inin mammalsmammals [[29,30],29,30], andand in inDrosophila Drosophilathe the adaptor adaptor protein protein GUK-holder GUK-holder (Gukh) (Gukh) links links Scrib Scrib and Dlg1and Dlg1 [31]. In[31]. addition, In addition, multiple multiple proteins proteins directly directly interact intera to eachct ofto theeach core of componentsthe core components of the Scribble of the module Scribble in manymodule di ffinerent many cellular different contexts cellular and contexts processes, and and processes, the number and ofthe interacting number of proteins interacting is increasingly proteins is growingincreasingly [4]. Amonggrowing these [4]. proteins Among are th includedese proteins regulators are included of different regulato typesrs of of cell different polarity,cytoskeleton, types of cell vesicularpolarity, tracytoskeleton,fficking, tissue vesicular growth, trafficking, as well as tissue cell proliferation, growth, as well survival, as cell and proliferation, migration [survival,3,4]. Some and of thesemigration proteins [3,4]. might Some be of potentialthese proteins novel might regulators be po oftential asymmetric novel regulators neuroblast of (NB)asymmetric division, neuroblast another process(NB) division, in which another the Scribble process module in which isalso the Scribble operative. module is also operative. 2. Beyond Epithelia: The Scribble Polarity Module in Neuroblasts 2. Beyond Epithelia: The Scribble