| A Drosophila compound eye consists of hundreds of small units named ommatidia. Each ommatidium contains eight photoreceptor cells wrapped around a central lumen forming a tube-like structure. The formation and expansion of the central lumen rely on the apical secretion of the extracellular matrix protein Eyes shut (EYS), and the assistance of its binding partner, the transmembrane protein Prominin (Prom). Failure of proper lumen formation causes defects in the separation and positioning of the light sensing organelles of photoreceptor cells, the rhabdomeres, and lead to lack of optomotor responses.;Though morphologically distinct and representing two fundamental types of photoreceptors, Drosophila and human photoreceptor cells share conserved rhodopsin genes and transcription factors. In this study we demonstrate that the two key structural proteins EYS and Prom, which are responsible for driving morphogenic changes, are functionally conserved between Drosophila and human. Importantly, a mutant form of human Prom which causes human retinal degeneration diseases also lead to similar membrane disruption phenotypes in Drosophila. Our work reveals the existence of a common ancestral cellular mechanism to create and model the apical membranes of the two fundamental types of photoreceptors into their respective phototransduction compartment. Utilizing an eys, prom trans-heterozygous (EP-TH) mutant as a sensitive background, we performed a genetic screen to identify other genes involved in retinal lumen formation. From our screen we uncovered that an actin and non-muscle myosin II machinery is required for retinal lumen formation. The actomyosin network likely provides a contractile pulling force from inside the photoreceptor cell, which coordinates with the pushing force provided by EYS and Prom from the extracellular space to initiate and to expand the lumen. Our results demonstrate actomyosin machinery as a novel mechanism for lumen formation.;During our lumen formation studies, we found that piperazine-N,N'-bis(ethanesulfonic acid) (PIPES), a commonly used buffer for tissue fixation, causes severe lumen and cell morphology artifacts in Drosophila eyes. In contrast, cell and lumen morphology in tissues fixed in PBS buffer is consistent with the transmission electron microscopy and live imaging data. We suggest that PIPES buffer should be used with caution in tissue fixation, especially when studying lumen-containing tissues. |
