RPE Cells Engulf Microvesicles Secreted by Degenerating Rod Photoreceptors

Abstract

Rhodopsin is mislocalized to the inner segment plasma membrane (IS PM) in various blinding disorders including autosomal-dominant retinitis pigmentosa caused by class I rhodopsin mutations. In these disorders, rhodopsin-laden microvesicles are secreted into the extracellular milieu by afflicted photoreceptor cells. Using a Xenopus laevis model expressing class I mutant rhodopsin or Na 1 /K 1-ATPase (NKA) fused to Dendra2, we fluorescently labeled the microvesicles and found retinal pigment epithelial (RPE) cells are capable of engulfing microvesicles containing rhodopsin. A unique sorting mechanism allows class I mutant rhodopsin, but not NKA, to be packaged into the microvesicles. Under normal physiological conditions, NKA is not shed as Significance Statement Rhodopsin mislocalization is a common cause of photoreceptor degeneration in inherited blinding disorders. In this study, we describe a novel mechanism of removing rhodopsin molecules from inner segment plasma membrane (IS PM). Mislocalized rhodopsin is packaged into microvesicles, which are secreted into the interphotoreceptor space and cleared via engulfment by retinal pigment epithelial (RPE) cells. While IS PM-mislocalized rhodopsin is specifically packaged into microvesicles, Na 1 /K 1-ATPase a-subunit, an IS PM resident protein, was not sorted into vesicles under either pathologic or normal physiological conditions. Interaction between photoreceptor and RPE cells is critical for maintaining visual function, and its alteration can lead to compromised vision. This study provides novel insights into photoreceptor-RPE cell interaction in inherited blinding disorders.

Release of VAMP5-positive extracellular vesicles by retinal Müller gliain vivo

“…Microvesicle release from photoreceptors was reported in vivo (Ropelewski and Imanishi, 2020). Here, we provide the first direct comparison of neuronal and glial secretomes and of the respective cell lysates.…”

“…Few studies characterized EVs secreted by other types of retinal cells in vitro including precursor cells (Zhou et al, 2018), astrocytes (Hajrasouliha et al, 2013), retinal ganglion cells (Wang et al, 2021) and photoreceptors (Kalargyrou et al, 2021). Microvesicle release from photoreceptors was reported in vivo (Ropelewski and Imanishi, 2020). Here, we provide the first direct comparison of neuronal and glial secretomes and of the respective cell lysates.…”

Release of VAMP5‐positive extracellular vesicles by retinal Müller glia in vivo

“…Few studies characterized EVs secreted by other types of retinal cells in vitro including precursor cells (Zhou et al., 2018 ), astrocytes (Hajrasouliha et al., 2013 ), retinal ganglion cells (Wang et al., 2021 ) and photoreceptors (Kalargyrou et al., 2021 ). Microvesicle release from photoreceptors was reported in vivo (Ropelewski & Imanishi, 2020 ). Here, we provide the first direct comparison of neuronal and glial secretomes and of the respective cell lysates.…”

Microvesicle release from inner segments of healthy photoreceptors is a conserved phenomenon in mammalian species

“…Why would such a mechanism exist in addition to the classical endolysosomal pathway? One possible explanation can be found in two studies of transgenic frogs expressing mutant rhodopsin ( Ropelewski and Imanishi, 2019 , 2020 ). The authors noted that both microvesicle release and endolysosomal degradation are capable of removing mutant rhodopsin from the inner segment plasma membrane.…”

“…Intriguingly, each of the proteins mutated in the latter group is implicated in supporting protein trafficking, including the trafficking of the visual pigment rhodopsin from the inner to the outer segment. A comparable vesicular accumulation around inner segments takes place in mice, rabbits, rats and frogs transgenically expressing various rhodopsin mutants ( Li et al, 1996 ; Kondo et al, 2009 ; Concepcion and Chen, 2010 ; Lodowski et al, 2013 ; LaVail et al, 2018 ; Ropelewski and Imanishi, 2020 ). It has been suggested that, at least in the case of mutant rhodopsin, this vesicular release serves as a safeguard mechanism counteracting the pathological accumulation of mislocalized protein in the inner segment ( Lodowski et al, 2013 ).…”