Gene 277 (2001) 83–100 www.elsevier.com/locate/gene Review Epithelial biology: lessons from Caenorhabditis elegans Gre´goire Michaux, Renaud Legouis1, Michel Labouesse* Institut de Ge´ne´tique et de Biologie Mole´culaire et Cellulaire, CNRS /INSERM /ULP, BP. 163, F-67404 Illkirch Cedex, C.U. de Strasbourg, Strasbourg, France Received 4 May 2001; received in revised form 17 August 2001; accepted 4 September 2001 Received by A.J. van Wijnen Abstract Epithelial cells are essential and abundant in all multicellular animals where their dynamic cell shape changes orchestrate morphogenesis of the embryo and individual organs. Genetic analysis in the simple nematode Caenorhabditis elegans provides some clues to the mechan- isms that are involved in specifying epithelial cell fates and in controlling specific epithelial processes such as junction assembly, trafficking or cell fusion and cell adhesion. Here we review recent findings concerning C. elegans epithelial cells, focusing in particular on epithelial polarity, and transcriptional control. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Caenorhabditis elegans; Epithelial cell; Differentiation; Morphogenesis; Adherens junction; Hemidesmosome; Extracellular matrix; Trafficking; Cell fusion; Transcription 1. Introduction transcription factors control the onset of ‘epithelialisation’ by switching on the expression of proteins that establish cell Epithelial cells play an essential role during development polarity. Similarly, we do not know, except in a few cases and adult life by shaping organs, and by acting as a selective such as branching of the tracheal system (Metzger and Kras- barrier to regulate the exchange of ions, growth factors or now, 1999) and dorsal closure in Drosophila (Noselli and nutrients coming from the outside environment (Yeaman et Agnes, 1999), which genes induce major morphogenetic al., 1999). These functions rely on proper cell fate specifica- processes. In addition, the detailed genetic analysis of the tion and on the acquisition of a polarised phenotype. Epithe- mechanisms underlying morphogenesis is a relatively new lial cells can also easily become cancerous and a direct link area, and what we know comes mainly from the fly. between tumorigenesis, loss of cell polarity and adhesion has The nematode Caenorhabditis elegans provides another long been noted (Thiery et al., 1988; Behrens et al., 1989). powerful system to address the questions raised above Despite their importance and widespread occurrence, using a genetic approach in the integrated context of a many aspects concerning the biology of epithelial cells are live animal. Here we highlight recent progress in the analy- not fully understood. For instance, we do not know if specific sis of different aspects of C. elegans epithelial biology. We first discuss specification of epithelial cell fates in the Abbreviations: AC, Anchor Cell; ADAM, A Disintegrin And Metallo- embryo (Section 3). Then we review how apico-basal proteinase; AJ, Adherens Junction; CeAJ, C. elegans Apical Junction; polarity is established and maintained (Sections 4 and 7), DTC, Distal Tip Cell; DU, Dorsal Uterine (precursor); EGF, Epidermal how the epidermis becomes connected to the underlying Growth Factor; FNIII, FibroNectin type III; GFP, Green Fluorescent muscles (Section 5), and how the basal lamina influences Protein; GPI, GlycoPhosphoInositol; GST, Glutathione S-Transferase; IF, morphogenesis of internal epithelia (gonad, intestine and Intermediate Filament; LC, Linker Cell; LRR, Leucine Rich Repeat; MAGUK, Membrane-Associated Guanylate Kinase; MAP, Mitogen Acti- pharynx) (Sections 6, 8 and 9). Section 10 is devoted to vating Protein; MDCK, Madin–Darby Canine Kidney; PDZ, PSD95/Dlg/ pattern formation and morphogenesis of the egg-laying ZO-1; SJ, Septate Junction; sujn, spermatheca uterine junction (uterine system (uterus and vulva). valve); TJ, Tight Junction; ut, uterine toroid; utse, uterine-seam (cell); uv, uterine vulva (cell); VPC, Vulval Precursor Cell; VU, Ventral Uterine (precursor) 2. Background anatomy * Corresponding author. Tel.: 133-3-8865-3393; fax: 133-3-8865-3201. E-mail address: [email protected] (M. Labouesse). 1 Present address: CGM-CNRS, Avenue de la Terrasse, 91198 Gif/Yvette There are three broad categories of epithelial cells in C. Cedex, France. elegans: those that contribute to a classical epithelium by 0378-1119/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0378-1119(01)00700-4 84 G. Michaux et al. / Gene 277 (2001) 83–100 Table 1 Phenotypes associated with some genes acting in epithelial cellsa Tissue Process Gene Phenotype Refs. Animal Cell Major epidermis Specification elt-1 Emb lethal Absent 1 Differentiation lin-26, elt-5/6 Emb/larval lethal Ab. cells/defective elongation 2–5 Junction formation let-413, dlg-1, ajm-1 Emb lethal No elongation/ab. junctions 6–9 Morphogenesis hmr-1/hmp-1/hmp-2, let-502 Emb lethal No enclosure and/or no elongation 10–12 Connection with muscles mup-4, mua-3, let-805 Emb/larval lethal No muscle anchoring/ab 13–17 hemidesmosomes Trafficking che-14, lrp-1 Part larval lethal Cuticle defects 18,19 Vulva Specification Ras/MAPK pathway Egl Vul or Muv 20 Morphogenesis sqv-1, sqv-7, sqv-8 Egl or Ste Squashed vulva 21,22 Vulva/Uterus Connection cog-2, lin-11 Egl No connection 23,24 Intestine Specification EDR (end-1 1 end-3) Emb lethal* Absent 25–27 Differentiation elt-2 Larval lethal Ab. cells/no food intake 28 Morphogenesis lin-12/glp-1 pathway Emb lethal# No rotation 29 a This table provides a few examples of the terminal phenotypes associated with mutations affecting epithelial cells but is not an exhaustive list of all genes acting in epithelia. Emb: embryonic; Egl: egg-laying defective; Ste: sterile; Vul: vulvaless; Muv: multivulva; Ab: abnormal; Part: partial; EDR: endoderm determining region (so far only large chromosomal deficiencies removing both end-1 and end-3 lead to absence of the intestine; the intestinal phenotype of those deficiencies can be rescued by introduction of an end-1 or end-3 transgene); MAPK: mitogen associated protein kinase. *Lethality associated with deficiencies removing the EDR are probably linked to the size of the deficiency. #Mutants in the lin-12/glp-1 pathway affect many other processes which contribute to embryonic lethality. 1 (Page et al., 1997); 2 (Koh and Rothman, 2001); 3 (Labouesse et al., 1994); (Labouesse et al., 1996); 5 (Quintin et al., 2001); 6 (Bossinger et al., 2001); 7 (Koeppen et al., 2001); 8 (Legouis et al., 2000); 9 (McMahon et al., 2001); 10 (Costa et al., 1998); 11 (Raich et al., 1999); 12 (Wissmann et al., 1997); 13 (Bercher et al., 2001); 14 (Gatewood and Bucher, 1997); 15 (Hong et al., 2001); 16 (Hresko et al., 1999); 17 (Plenefisch et al., 2000); 18 (Michaux et al., 2000); 19 (Yochem et al., 1999); 20 (Sternberg and Han, 1998); 21 (Herman and Horvitz, 1999); 22 (Herman et al., 1999); 23 (Hanna-Rose and Han, 1999); 24 (Newman et al., 1999); 25 (Zhu et al., 1997); 26 (Zhu et al., 1998); 27 (Maduro et al., 2001); 28 (Fukushige et al., 1998); 29 (Hermann et al., 2000). forming either a sheet (i.e. the epidermis and vulva) or a principally on the first category, particularly on epidermal tube (i.e. the intestine and pharynx); those that are isolated cells which have been well-characterised in nematodes (i.e. the excretory cell); those that form a contractile myoe- (Fig. 1A). Epidermal cells are often called hypodermal pithelial organ (i.e. the spermatheca) (White, 1988). Muta- because they secrete a collageneous cuticle at their apical tions that affect the differentiation, structure, surface, which acts as an exoskeleton to maintain the shape morphogenesis or function of epithelial cells generally of the animal and to anchor muscles. Three classes of result in lethality (see Table 1). This review will focus epidermal cells can be distinguished: cells which cover Fig. 1. Epithelial cells in Caenorhabditis elegans. (A) Schematic drawing illustrating the relative positions and sizes of the four main epithelial tissues that will be discussed in this review, at mid-embryogenesis (1.5-fold embryo) and in the adult. (B) Confocal picture of an embryo showing internal epithelia (pharynx and intestine) stained with antibodies against PAR-3 (green) which marks the lumen, and the MH27 monoclonal antibody against AJM-1 (red) which marks junctions (the inset on the right corresponds to an enlargement of the boxed area). (C) Confocal picture of a young transgenic larva showing internal epithelia (pharynx and intestine) expressing a DLG-1::GFP translational fusion. Anterior to the left and dorsal up in A-B; anterior in the right-hand corner in C. (D) In the epidermis (simplified diagram on the left), confocal and electron microscopy analyzes have suggested that junctions (example on the right) comprise two independent units (McMahon et al., 2001). The apical-most unit, which is critical to anchor the actin cytoskeleton, corresponds to the classical cadherin-catenin complex encoded by the genes hmr-1 (E-cadherin), hmp-2 (b-catenin) and hmp-1 (a-catenin). The other unit corresponds to a complex formed by the MAGUK protein DLG-1, AJM-1 and possibly other proteins. The electron dense structure visible by electron microscopy (enlarged in pink inset) coincides at least with the AJM-1/DLG-1 unit, and might even be restricted to it. Eliminating the function of a gene in one unit appears to leave the other unit largely unaffected. There are other known proteins at the CeAJ, such as the APC-related protein APR-1 which is essential for the process of ventral enclosure (see Fig. 3A) (Hoier et al., 2000) or small GTPases (Chen and Lim, 1994), however it is not known whether these protein colocalise with HMP-1 or with AJM-1. LET-413 is a basolateral protein required to coalesce both units at a sub-apical position during epidermal differentiation. (E) Muscle cells are tightly connected across the epidermis to the external cuticle, resulting in a strong flattening of the epidermis in areas of muscle contact (diagram on the left).