Abstract
Purpose :
Huntington’s disease (HD) is an inherited neurodegenerative disease caused by the expansion of a CAG repeat (PolyQ) in the first exon of the huntingtin gene (HTTex1). Healthy individuals have about 20 repeats whereas HD patients have more than 36 polyQs. This mutant huntingtin protein (mHTT) forms aggregates in cortical and striatal neurons, causing damage and death. Studies from human fetuses carrying the HD mutation have shown abnormalities in the developing cortex, including mislocalization of mHTT and junctional complex proteins, affecting neural progenitor cell polarity. HD patients also exhibit visual deficits. The purpose of this study is to use HD mouse models to examine how mutant huntingtin affects the retinae.
Methods :
Retinal cryosections and RPE wholemounts from transgenic R6/1 and zQ175 knock-in (zQ175 KI) HD mouse models were processed for immunocytochemistry to examine the presence of huntingtin aggregates as well as position of cell junction and cell polarity proteins (ZO-1, Crb2 and Par3). Cones were visualized using cone markers, peanut agglutinin, and cone arrestin.
Results :
The zQ175 KI and R6/1 retinal sections were labeled with PHP1 antibody, which preferentially binds aggregated mHTTex1. The PHP1 signal was present in all retinal layers and particularly abundant in the ganglion cells. PHP1 predominantly labels small cytoplasmic aggregates in zQ175 KI sections whereas intranuclear aggregates were commonly seen in R6/1 sections. No staining was seen in the control retinas. The cell polarity proteins were highly aligned at the outer limiting membrane in the control sections but not in that of R6/1 and zQ175 KI. ZO-1, which is part of the junctional complex in the polarized RPE, showed the cobblestone borders in the control. This pattern was disrupted in the R6/1 RPE. We found the cone markers and cone numbers were significantly decreased in the mutant retinae compared with controls.
Conclusions :
mHTT aggregates were detected in retinae of the HD mouse models. In addition, tight junction proteins were disrupted in the retinal outer limiting membrane and RPE cells of HD mouse models. These data imply a general role of HTT in the establishment of cell polarity. mHTT also has a negative impact on cone photoreceptors. The eye may serve as a useful system to track disease progression and to test therapeutic intervention strategies.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.