Protein Equilibration Through Somatic Ring Canals in Drosophila

Abstract

Although intercellular bridges resulting from incomplete cytokinesis were discovered in somatic Drosophila tissues decades ago, the impact of these structures on intercellular communication and tissue biology is largely unknown. In this work, we demonstrate that the ~250 nm diameter somatic ring canals permit diffusion of cytoplasmic contents between connected cells and across mitotic clone boundaries, and enable the equilibration of protein between transcriptionally mosaic follicle cells in the Drosophila ovary. We obtained similar, though more restricted, results in the larval imaginal discs. Our work illustrates the lack of cytoplasmic autonomy in these tissues and suggests a role for somatic ring canals in promoting homogeneous protein expression within the tissue.

Bridging the divide

“…Thus, both GFP and GFP-tagged endogenous proteins move freely through somatic RCs of multiple tissues. 13 Our model of diffusion-mediated exchange is supported by analysis of RC structure by electron microscopy (EM), which reveals no structural elements that could regulate movement between cells. Immediately inside the RC backbone is a layer of circumferential filamentous actin and, in most cases, a layer of rough ER (rER).…”

“…12,13 The results suggest somatic RCs can provide cells with a mechanism to equilibrate protein levels and compensate for disparities in transcription between connected cells.…”

“…Immediately inside the RC backbone is a layer of circumferential filamentous actin and, in most cases, a layer of rough ER (rER). 11,13 Previous measurements of follicle cell RCs reported the limiting diameter of the filamentous layer to be 230-260 nm, but did not account for additional occlusion by the rER. 11 We derived the effective inside diameter of follicle cell RCs by using a computational model of diffusion that recapitulates the observed exchange of PAGFP in vivo, and arrived at 80-220 nm (Fig.…”

“…Indeed, a co-stain for GFP transcript revealed that only individual or small clusters of cells contained significant amounts of transcript, meaning that the much broader distribution of protein resulted from protein diffusing out of the cells of origin. 13 Though these experiments used the exaggerated mosaicism afforded by the Gal4/UAS system, they demonstrate that RCs do allow the equilibration of protein among cells with different levels of transcription. It is therefore likely that other proteins capable of intercellular diffusion would similarly equilibrate within syncytia regardless of transcriptional disparities, potentially contributing to the overall health of these epithelial cells.…”

“…Using FLIP and photoactivation experiments, we confirmed that these patches of expression correspond to the underlying syncytia. 13 Follicle cell syncytia, evaluated directly by PAGFP activation experiments and indirectly by mosaic GFP expression, have no consistent configuration, orientation, or polarity with respect to egg chamber axes or other morphological features. For example, mosaic GFP patches frequently cross the domains of rhomboid 22 and Broad 23 expression in stage 10a and 10b egg chambers, emphasizing the cellular autonomy of the pathways responsible for dorsal appendage formation (McLean P, Du J, Cooley L; unpublished).…”