Abstract
Purpose :
In certain models of photoreceptor degeneration, microvesicles containing rhodopsin can be released from photoreceptor inner segments and accumulate in the extracellular space surrounding these cells. In this work, we sought to address the contribution of rhodopsin to this microvesicle release.
Methods :
IFT20 is thought to be directly involved in the transport of rhodopsin from the Golgi to the outer segment. To study the role of mislocalized rhodopsin in microvesicle release, we generated a rod-specific knockout of IFT20 in mice. Notably, the depletion of IFT20 protein in this line does not complete until after photoreceptor outer segments are fully developed, allowing us to study microvesicle release in mature photoreceptors.
Results :
We found that in the rod-specific IFT20 knockout, photoreceptors rapidly degenerate after IFT20 protein is fully turned over. At the early stages of degeneration, we observe a massive accumulation of microvesicles surrounding photoreceptor inner segments, with several examples in which microvesicles were captured in the process of budding from the inner segment plasma membrane. Using quantitative immunogold labeling, we find that loss of IFT20 leads to an accumulation of rhodopsin in the inner segment plasma membrane as well as these microvesicles. Strikingly, the labeling density of rhodopsin in microvesicles is nearly three times higher than that of the inner segment plasma membrane, indicating that rhodopsin is concentrated in microvesicles prior to release. We next show that this microvesicle release is not simply a consequence of photoreceptor degeneration, but rather that it requires gross mislocalization of rhodopsin. Strikingly, microvesicle release from the inner segment does not occur exclusively under pathological conditions as rhodopsin-containing microvesicles are also released in small quantities from inner segments of normal, healthy photoreceptors.
Conclusions :
This work indicates the existence of a fundamental process whereby both healthy and diseased photoreceptors release mistrafficked rhodopsin as microvesicles into the interphotoreceptor space. Furthermore, rhodopsin molecules are concentrated from the plasma membrane into these microvesicles before release, suggesting that the rhodopsin molecule plays a specific role in this process.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.