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Y.-W. Peng, M. Zallocchi, W. Wang, D. Cosgrove; Defects in Rod Protein Translocation in Usher Syndrome Type IIa. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4460.
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© ARVO (1962-2015); The Authors (2016-present)
Usher syndrome type IIa is the most common of the Usher syndromes. The gene responsible encodes at least two different protein isoforms called usherin. Previously, we have reported that the usherin hypomorph mice show late-onset slow progressive retinal degeneration and markedly slowed rod protein translocation(Peng et al., ARVO Abstract 2007, 2008). Here we explore the specific functional changes of protein translocation in usherin hypomorph rods in detail.
Immunohistochemistry and immunoblotting were used to study the light-induced protein translocation.
Protein translocation in rods can be observed only when the light intensity exceeds a critical threshold level. After 200 lux light adaptation for 10 min in dark adapted wild type mice, significant amount of transducin had been translocated to the inner segments and synaptic terminals. Under these same conditions in the usherin hypomorph mice, most of the transducin was still in the outer segments. There was almost no staining of transducin in the synaptic terminals of usherin hypomorph mice. After 500 lux light adaptation for 1 hour, transducin in usherin hypomorph rods was translocated to the synaptic terminals, indicating the activation threshold of transducin translocation in the usherin hypomorph rods had been shifted to a higher level. Compared to wild type mice under the same conditions, translocation of transducin in usherin hypomorph rods showed a marked delay. More importantly, such delays of protein translocation were also detected in 6-week-old usherin hypomorph mice when there was no sign of photoreceptor degeneration, suggesting that the delay is caused by the dysfunction of usherin rather than a secondary effect of rod degeneration. Light adaptation could induce/accelerate the degeneration of rods with translocation defects. Under dark adaptation, transducin in the usherin hypomorph rods was translocated back to the outer segments much faster than that in the wild type retina.
The delay of rod protein translocation in usherin hypomorph mice is likely due to a shift of activation threshold to a higher level. Elevated threshold means that the phototransduction in the rod outer segments continues to work in high sensitive mode under bright light. This may increase the metabolic stress in rods under conditions of bright illumination; and may explain why these rods are vulnerable to light-induced degeneration. The faster return of transducin to the outer segments on dark adaptation suggests that the affinity for transducin in the outer segments of usherin hypomorph rods is higher than that in the wild type rods, and thus requiring higher light intensity to activate the translocation.
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