Building Castles in the Eye

If you’ve ever seen a building collapse, you’ll be aware of the damage a lack of structural integrity can cause. Similar principles apply within the eye, where stability is maintained by the connecting cilium linking the outer segment of photoreceptors with the cell body. Structural defects of the connecting cilium have already been associated with retinal degeneration; however, its nanoscale molecular composition, assembly, and function are relatively unknown. New insight has been provided by a team from the University of Geneva, Switzerland, who identified a four-protein molecular zipper whose loss removes a protective structural foundation, resulting in photoreceptor death and retinal degeneration (1). Focusing on the RP28 retinitis pigmentosa subtype associated with recessive mutations in the human FAM161A gene (which produces one of the proteins in the molecular zipper), the researchers sought to understand when and where defects arise, because knowing how everything collapses may provide new ways for vision to be rebuilt and strengthened.

The research group leaders, Virginie Hamel and Paul Guichard, are no strangers to analyzing biological architecture. “Our laboratory specializes in structural cell biology. Of particular interest to us is the centriole, a microtubule-based organelle found at the core of mitotic division as well as cilium formation,” explains Hamel. “We recently unveiled the presence of a structural element – the inner scaffold – within the centriole and identified components of this structure using expansion microscopy, finding that it maintains the centriole’s cohesion. We completed this study because we were intrigued that all of the inner scaffold components that have been identified are localized to the connecting cilium within photoreceptors, and mutations within these proteins resulted in photoreceptor degeneration. This led us to hypothesize that a similar inner scaffold structure could be present in photoreceptor cells and might explain the phenotype observed in mouse models.”

A key aspect of the study was the use of expansion microscopy for retinal tissue, which provided the research team with a new and easy way of performing high-resolution imaging. This allowed them to observe protein localization at nanoscale resolution, using only a simple wide-field microscope present in most laboratories. This powerful tool has the ability to complement gene therapy approaches, for example by enabling researchers to monitor the impact of adding back inner scaffold components. Expansion microscopy may make rescuing defects within the connecting cilium a possibility through gene therapy approaches to restore the molecular zipper, ensure the structural integrity of the microtubules, and prevent photoreceptor death.

Hamel and her team will be leading the charge in this area, focusing on the FAM161A protein. “One area of our future research will be the continued dedication to understanding retinal degeneration. We are planning to continue our great collaboration with the laboratories of Corinne Kostic and Yvan Arsenijevic in Lausanne, Switzerland, to explore gene therapy for FAM161A as a therapeutic tool for retinitis pigmentosa RP28.” She continues, “Of course, this is a mid-term goal and there are still some key steps to achieve, but we would be very happy to contribute to this effort using expansion microscopy. While we do this, we would also like to explore and better understand the retina’s molecular composition and organization using ultrastructure expansion microscopy.”