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Shannon M. Conley, Dibyendu Chakraborty, Muna I. Naash; A Unique Approach To The Study Of The Function Of ROM-1 In The Absence Of RDS. Invest. Ophthalmol. Vis. Sci. 2012;53(14):1632.
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© ARVO (1962-2015); The Authors (2016-present)
Rod outer segment membrane protein-1 (ROM-1) is a tetraspanin found in the disc rims of rods and cones. ROM-1 interacts with its homolog retinal degeneration slow (RDS) to promote outer segment (OS) morphogenesis, structural maintenance and disc sizing. Due to the nearly undetectable levels of ROM-1 in the absence of RDS (rds-/-), it has been very difficult to study its precise function which is the goal of this project.
Two models were used; 1) the rhodopsin knockout (rho-/-) was bred into the rds-/- background to create rho-/-/rds-/- double knockout mice, and 2) knock-in mice were generated expressing a chimeric protein comprising the body of ROM-1 and the RDS C-terminus (RRCT) in the RDS genetic locus. Retinal structure and function were analyzed by ERG, light and electron microscopy, and immunofluorescence; while complex formation was analyzed by immunoprecipitation and velocity sedimentation.
In contrast to rds-/- retinas in which ROM-1 is not detectable, rho-/-/rds-/- retinas exhibit significant levels of ROM-1. ROM-1 is found at the tip of the connecting cilium and is not retained in the inner segment. Furthermore, it forms tetramers but not higher-order oligomeric complexes. However, rho-/-/rds-/- photoreceptors exhibit no signs of OS disc formation in contrast to the rho-/-/rds+/+ in which nascent discs/rims are seen although no mature discs are formed. In contrast, in mice homozygous for the RRCT allele, (i.e. two ROM-1 alleles, two RRCT alleles, zero wild-type RDS), overall retinal structure is better than the rho-/-, rho-/-/rds-/-, and rds-/-. RRCT mice exhibit very small grossly abnormal OSs that are nonetheless characterized by swirls of disc/rim membrane and some visual function. Both RRCT and ROM-1 proteins are found properly localized to the OS in RRCT knock-in mice.
These results demonstrate that ROM-1 is stable and can independently traffic to the OS in the absence of RDS. We also show that the RRCT protein but not ROM-1 is capable of initiating OS disc formation. This suggests 1) that the RDS C-terminus is required for the initiation of disc formation while the body of RDS and RDS oligomerization are required for proper disc maturation, and 2) that ROM-1, which cannot form higher-order oligomers, can promote neither of these steps.
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