ARL2 regulates trafficking and expression of isoprenylated proteins and is crucial for development of photoreceptor outer segments

Abstract

Purpose : ARL2 is a member of the ARL small GTPase family. ARL2 interacts with PDEδ, an isoprenyl group binding protein mediating transport of isoprenylated proteins from photoreceptor inner segments to outer segments. Mutation of PDE6D, the gene encoding PDEδ, is associated with Joubert syndrome. ARL2 also interacts BART. Mutations of the BART gene cause autosomal recessive rod-cone dystrophies. In retina, BART is localized to the basal body of photoreceptors. We generated a retina-specific Arl2 knockout mouse to identify the role of ARL2 in retina.

Methods : The mouse ES cells with floxed Alr2 were generated by gene targeting using homologous recombination. Arl2^flox/flox mice derived from the mutant ES cells were bred with Six3-Cre mice to generate a retina-specific Arl2 knockout mouse. Retinal funciton of the mutant mice was assessed by ERG recordings. Scotopic and photopic electroretinography (ERG) was performed on P14 Arl2^-/- mice. IHC and western blot were performed on retina taken from P14 mice using antibodies against rhodopsin, PDE6, cone opsins, GRK1, and INPP5E.

Results : Deletion of Arl2 in mouse retina (Arl2^ret-/-) caused microphthalmia. Severe retinal degeneration was observed in Arl2^ret-/- as early as P12. At P30, photoreceptors were completely degenerated and the remaining retinal cells were disorganized. Scotopic and photopic ERGs were nearly unrecordable in Arl2^ret-/- mice at P14. The length of outer segments was dramatically reduced in Arl2^ret-/-retina. Expression of rhodopsin and cone opsins were significantly downregulated, although their trafficking to outer segments appeared normal. Isoprenylated GRK1 and PDE6, were mistrafficked to inner segments and sypnatic terminals in Arl2^ret-/-. While the expression of PDE6 was reduced, the expression of GRK1 and INPP5E was unexpectedly increased in Arl2^ret-/- retina.

Conclusions : ARL2 plays a critical role in retinal development and function.Within photoreceptors, ARL2 regulates trafficking of isoprenylated proteins and is essential for formation and maintenance of photoreceptor outer segments. Additionally, ARL2 is required for proper expression of rhodopsin, GRK1 and INPP5E in retina.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.

 

Isoprenylated PDE6 and GRK1 are mistrafficked in Arl2^Ret-/- retina.

 

Expression of isoprenylated INPP5E is increased in Arl2^ret-/- retina.