Role of Spectraplakin in Drosophila Photoreceptor Morphogenesis

BACKGROUND:Crumbs (Crb), a cell polarity gene, has been shown to provide a positional cue for the apical membrane domain and adherens junction during Drosophila photoreceptor morphogenesis. It has recently been found that stable microtubules in developing Drosophila photoreceptors were linked to Crb localization. Coordinated interactions between microtubule and actin cytoskeletons are involved in many polarized cellular processes. Since Spectraplakin is able to bind both microtubule and actin cytoskeletons, the role of Spectraplakin was analyzed in the regulations of apical Crb domain in developing Drosophila photoreceptors. METHODOLOGY/PRINCIPAL FINDINGS:The localization pattern of Spectraplakin in developing pupal photoreceptors showed a unique intracellular distribution. Spectraplakin localized at rhabdomere terminal web which is at the basal side of the apical Crb or rhabdomere, and in between the adherens junctions. The spectraplakin mutant photoreceptors showed dramatic mislocalizations of Crb, adherens junctions, and the stable microtubules. This role of Spectraplakin in Crb and adherens junction regulation was further supported by spectraplakin's gain-of-function phenotype. Spectraplakin overexpression in photoreceptors caused a cell polarity defect including dramatic mislocalization of Crb, adherens junctions and the stable microtubules in the developing photoreceptors. Furthermore, a strong genetic interaction between spectraplakin and crb was found using a genetic modifier test. CONCLUSIONS/SIGNIFICANCE:In summary, we found a unique localization of Spectraplakin in photoreceptors, and identified the role of spectraplakin in the regulation of the apical Crb domain and adherens junctions through genetic mutational analysis. Our data suggest that Spectraplakin, an actin-microtubule cross-linker, is essential in the apical and adherens junction controls during the photoreceptors morphogenesis

Shot is essential for photoreceptor morphogenesis in the mid-stage developing pupal eyes.

<p>(A and B) Mid-stage pupal eyes (45% pd) with <i>shot³</i> null mutant clones marked by the absence of the GFP and dotted lines (green, A' and B'). Crb (red, A, arrow) was almost absent, and E-cad (blue, A, arrowhead) was mislocalized from the apical center toward the surrounded distal areas (arrow, A). Stable microtubules (Acetub, red) were also mislocalized in the <i>shot³</i> mutants (arrow, B).</p

Crb is essential for Shot localization in the mid-stage developing pupal eyes.

<p>(A) Pupal eyes (45% pupal stage) with <i>crb^11A22</i> null mutant clones marked by the absence of the GFP (green, A''). (A) Shot (red, A, arrow) and adherens junction (E-cad, blue, A', arrowhead) were mislocalized from apical to basal (A and A', arrow and arrowhead).</p

Genetic interactions between <i>crb</i> and <i>shot</i> in <i>Drosophila eye</i>.

<p>(A–B) Adult eye phenotypes of+/+(A), <i>shot³/+</i>(B), <i>GMR>Crb^intra</i>;+/+(C), and <i>GMR>Crb^intra</i>; <i>shot³/+</i>(D).</p

Morphogenesis of <i>Drosophila</i> pupal photoreceptors.

<p>(A) Side view of developing photoreceptors at 45% pd. The photoreceptors elongate from distal to proximal (arrow). (B) Cross-section of 45% pd pupal photoreceptors. Apical domain (green) localizes apical to adherens junction (red) in the center of a photoreceptor cluster. The E-cad localizes at adherens junction (red) which are more basal to the apical domain. The basolateral domains (black) are more basal to the adherens junction (red), and the acetylated-tubulin (blue) localizes at the outside from the adherens junctions (red). The rhabdomere (light blue) localizes at the apical to the apical domain (green).</p

Overexpression of Shot causes the mislocalization of apical domain, adherens junction, and stable microtubules.

<p>Pupal eyes (45% pd) with Shot overexpression driven by <i>GMR-GAL4</i> at 22°C were examined by Crb (apical domain marker, red, A–C), E-cad (adherens junction marker, A–E) and acetylated-tubulin (Acetub, stable microtubules marker, red, D–E). (A, D) control, <i>GMR-GAL4/+</i>, (B, E) <i>GMR-GAL4/UAS-ShotA-GFP</i>, (C) <i>GMR-GAL4/UAS-ShotC-GFP</i>.</p

Localization of Shot in <i>Drosophila</i> photoreceptors.

<p>(A–F) Localization of Shot in pupal photoreceptors (45% pd). (A and B) Shot (A and B, red) localizes in between the E-cad (A' and B', blue), at the basal side of the aPKC (apical marker, A, green), and at the apical side of the microtubules (B, green). (C) Shot localizes at the basal side of phalloidin (Phal, rhabdomere marker, C and C''). (D–F) Schematic representation of mid-pupal photoreceptor and localization of Shot was labeled (orange). Shot (orange) localizes in between adherens junction (red), at the basal side of the apical Crumbs domain (green), at the apical side of the stable microtubule (blue), and at the basal side of the rhabdomere (light blue). (G) Localization of Shot in eye discs of third-instar larvae. Shot localizes at the basal side of the apical domain (aPKC, G'), and in between adherens junction (E-cad, G'').</p