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Adherens junction-dependent and -independent steps in the establishment of epithelial cell polarity in Drosophila.

Harris TJ, Peifer M - J. Cell Biol. (2004)

Bottom Line: We found apical accumulation of both Drosophila E-Cadherin (DE-Cad) and the apical cue Bazooka (Baz) as cells first form.Some epithelial structures are retained, however.These structures maintain apical Baz, accumulate apical Crumbs, and organize polarized cytoskeletons, but display abnormal cell morphology and fail to segregate the basolateral cue Discs large from the apical domain.

View Article: PubMed Central - PubMed

Affiliation: Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.

ABSTRACT
Adherens junctions (AJs) are thought to be key landmarks for establishing epithelial cell polarity, but the origin of epithelial polarity in Drosophila remains unclear. Thus, we examined epithelial polarity establishment during early Drosophila development. We found apical accumulation of both Drosophila E-Cadherin (DE-Cad) and the apical cue Bazooka (Baz) as cells first form. Mutant analyses revealed that apical Baz accumulations can be established in the absence of AJs, whereas assembly of apical DE-Cad complexes requires Baz. Thus, Baz acts upstream of AJs during epithelial polarity establishment. During gastrulation the absence of AJs results in widespread cell dissociation and depolarization. Some epithelial structures are retained, however. These structures maintain apical Baz, accumulate apical Crumbs, and organize polarized cytoskeletons, but display abnormal cell morphology and fail to segregate the basolateral cue Discs large from the apical domain. Thus, although epithelial polarity develops in the absence of AJs, AJs play specific roles in maintaining epithelial architecture and segregating basolateral cues.

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Baz localizes correctly in armm/z and shgm/z mutants. (A) armm/z cross sections. Note apical Baz (red) during mid cellularization (arrow). DE-Cad (green) is cytoplasmic. Dlg (blue) shows normal furrows. In bottom panels, note that DE-Cad (green) has minimal associations with the plasma membrane stained with Dlg (red). (B) armm/z surface views. Baz complexes (red; arrowheads) resemble WT spot junctions. They overlap with Dlg (blue) but do not recruit DE-Cad (green). (C) armm/z surface views. Baz complexes (red; arrowhead) do not recruit Arm (blue) or α-cat (green). Insets show the colocalization of these proteins in WT spot junctions (arrowheads). (D) shgm/z cross sections. Note apical Baz (red) during mid cellularization (arrow). DE-Cad (gray) is cytoplasmic. Dlg (blue) shows normal furrows. In images of AJ proteins in armm/z and shgm/z mutants darker grays were converted to black, to detect any possible AJ protein accumulation by removing the low level cytoplasmic staining. Bars, 5 μm.
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fig2: Baz localizes correctly in armm/z and shgm/z mutants. (A) armm/z cross sections. Note apical Baz (red) during mid cellularization (arrow). DE-Cad (green) is cytoplasmic. Dlg (blue) shows normal furrows. In bottom panels, note that DE-Cad (green) has minimal associations with the plasma membrane stained with Dlg (red). (B) armm/z surface views. Baz complexes (red; arrowheads) resemble WT spot junctions. They overlap with Dlg (blue) but do not recruit DE-Cad (green). (C) armm/z surface views. Baz complexes (red; arrowhead) do not recruit Arm (blue) or α-cat (green). Insets show the colocalization of these proteins in WT spot junctions (arrowheads). (D) shgm/z cross sections. Note apical Baz (red) during mid cellularization (arrow). DE-Cad (gray) is cytoplasmic. Dlg (blue) shows normal furrows. In images of AJ proteins in armm/z and shgm/z mutants darker grays were converted to black, to detect any possible AJ protein accumulation by removing the low level cytoplasmic staining. Bars, 5 μm.

Mentions: Because AJs are thought to be key landmarks for establishing epithelial polarity, we tested the role of AJs in establishing the apical domain during cellularization. We examined maternal/zygotic mutants for armXP33 (referred to as armm/z mutants), a strong allele encoding a truncated protein that accumulates at much lower levels than wild-type (WT) Arm and lacks function in both AJs and Wingless signaling (Cox et al., 1996). These armm/z mutants have the most severe embryonic phenotype of any maternal/zygotic (m/z) mutant of a core AJ protein that can complete oogenesis. However, they undergo relatively normal syncytial development (Grevengoed et al., 2003), and although they have some early spindle attachment defects (McCartney et al., 2001), they effectively cellularize (Cox et al., 1996; Grevengoed et al., 2003). Thus, we investigated polarity establishment in armm/z mutants during mid to late cellularization. As in WT, Dlg localizes along the full length of the furrows, which appear morphologically normal, and to microvilli (Fig. 2 A). No AJs were detected—DE-Cad accumulates at much lower levels than in WT (Cox et al., 1996) and displays only weak cytoplasmic staining without plasma membrane enrichment (Fig. 2 A). Remarkably, apical Baz accumulations are established along the cellularization furrows in armm/z mutants (Fig. 2 A, arrows), with only weak Baz staining along lateral membranes. From the embryo surface, the apical Baz resembles the spot junctions seen in WT embryos (Fig. 2 B, arrowheads). However, DE-Cad, Arm, and α-cat show only weak staining and do not localize to Baz accumulations (Fig. 2, B and C, arrowheads), in contrast to WT (Fig. 1, C′ and D′; Fig. 2 C insets, arrowheads), suggesting little or no AJ function in the armm/z mutants.


Adherens junction-dependent and -independent steps in the establishment of epithelial cell polarity in Drosophila.

Harris TJ, Peifer M - J. Cell Biol. (2004)

Baz localizes correctly in armm/z and shgm/z mutants. (A) armm/z cross sections. Note apical Baz (red) during mid cellularization (arrow). DE-Cad (green) is cytoplasmic. Dlg (blue) shows normal furrows. In bottom panels, note that DE-Cad (green) has minimal associations with the plasma membrane stained with Dlg (red). (B) armm/z surface views. Baz complexes (red; arrowheads) resemble WT spot junctions. They overlap with Dlg (blue) but do not recruit DE-Cad (green). (C) armm/z surface views. Baz complexes (red; arrowhead) do not recruit Arm (blue) or α-cat (green). Insets show the colocalization of these proteins in WT spot junctions (arrowheads). (D) shgm/z cross sections. Note apical Baz (red) during mid cellularization (arrow). DE-Cad (gray) is cytoplasmic. Dlg (blue) shows normal furrows. In images of AJ proteins in armm/z and shgm/z mutants darker grays were converted to black, to detect any possible AJ protein accumulation by removing the low level cytoplasmic staining. Bars, 5 μm.
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fig2: Baz localizes correctly in armm/z and shgm/z mutants. (A) armm/z cross sections. Note apical Baz (red) during mid cellularization (arrow). DE-Cad (green) is cytoplasmic. Dlg (blue) shows normal furrows. In bottom panels, note that DE-Cad (green) has minimal associations with the plasma membrane stained with Dlg (red). (B) armm/z surface views. Baz complexes (red; arrowheads) resemble WT spot junctions. They overlap with Dlg (blue) but do not recruit DE-Cad (green). (C) armm/z surface views. Baz complexes (red; arrowhead) do not recruit Arm (blue) or α-cat (green). Insets show the colocalization of these proteins in WT spot junctions (arrowheads). (D) shgm/z cross sections. Note apical Baz (red) during mid cellularization (arrow). DE-Cad (gray) is cytoplasmic. Dlg (blue) shows normal furrows. In images of AJ proteins in armm/z and shgm/z mutants darker grays were converted to black, to detect any possible AJ protein accumulation by removing the low level cytoplasmic staining. Bars, 5 μm.
Mentions: Because AJs are thought to be key landmarks for establishing epithelial polarity, we tested the role of AJs in establishing the apical domain during cellularization. We examined maternal/zygotic mutants for armXP33 (referred to as armm/z mutants), a strong allele encoding a truncated protein that accumulates at much lower levels than wild-type (WT) Arm and lacks function in both AJs and Wingless signaling (Cox et al., 1996). These armm/z mutants have the most severe embryonic phenotype of any maternal/zygotic (m/z) mutant of a core AJ protein that can complete oogenesis. However, they undergo relatively normal syncytial development (Grevengoed et al., 2003), and although they have some early spindle attachment defects (McCartney et al., 2001), they effectively cellularize (Cox et al., 1996; Grevengoed et al., 2003). Thus, we investigated polarity establishment in armm/z mutants during mid to late cellularization. As in WT, Dlg localizes along the full length of the furrows, which appear morphologically normal, and to microvilli (Fig. 2 A). No AJs were detected—DE-Cad accumulates at much lower levels than in WT (Cox et al., 1996) and displays only weak cytoplasmic staining without plasma membrane enrichment (Fig. 2 A). Remarkably, apical Baz accumulations are established along the cellularization furrows in armm/z mutants (Fig. 2 A, arrows), with only weak Baz staining along lateral membranes. From the embryo surface, the apical Baz resembles the spot junctions seen in WT embryos (Fig. 2 B, arrowheads). However, DE-Cad, Arm, and α-cat show only weak staining and do not localize to Baz accumulations (Fig. 2, B and C, arrowheads), in contrast to WT (Fig. 1, C′ and D′; Fig. 2 C insets, arrowheads), suggesting little or no AJ function in the armm/z mutants.

Bottom Line: We found apical accumulation of both Drosophila E-Cadherin (DE-Cad) and the apical cue Bazooka (Baz) as cells first form.Some epithelial structures are retained, however.These structures maintain apical Baz, accumulate apical Crumbs, and organize polarized cytoskeletons, but display abnormal cell morphology and fail to segregate the basolateral cue Discs large from the apical domain.

View Article: PubMed Central - PubMed

Affiliation: Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.

ABSTRACT
Adherens junctions (AJs) are thought to be key landmarks for establishing epithelial cell polarity, but the origin of epithelial polarity in Drosophila remains unclear. Thus, we examined epithelial polarity establishment during early Drosophila development. We found apical accumulation of both Drosophila E-Cadherin (DE-Cad) and the apical cue Bazooka (Baz) as cells first form. Mutant analyses revealed that apical Baz accumulations can be established in the absence of AJs, whereas assembly of apical DE-Cad complexes requires Baz. Thus, Baz acts upstream of AJs during epithelial polarity establishment. During gastrulation the absence of AJs results in widespread cell dissociation and depolarization. Some epithelial structures are retained, however. These structures maintain apical Baz, accumulate apical Crumbs, and organize polarized cytoskeletons, but display abnormal cell morphology and fail to segregate the basolateral cue Discs large from the apical domain. Thus, although epithelial polarity develops in the absence of AJs, AJs play specific roles in maintaining epithelial architecture and segregating basolateral cues.

Show MeSH
Related in: MedlinePlus