Identification and Functional Analysis of the Vision-Specific BBS3 (ARL6) Long Isoform

Bardet-Biedl Syndrome (BBS) is a heterogeneous syndromic form of retinal degeneration. We have identified a novel transcript of a known BBS gene, BBS3 (ARL6), which includes an additional exon. This transcript, BBS3L, is evolutionally conserved and is expressed predominantly in the eye, suggesting a specialized role in vision. Using antisense oligonucleotide knockdown in zebrafish, we previously demonstrated that bbs3 knockdown results in the cardinal features of BBS in zebrafish, including defects to the ciliated Kupffer's Vesicle and delayed retrograde melanosome transport. Unlike bbs3, knockdown of bbs3L does not result in Kupffer's Vesicle or melanosome transport defects, rather its knockdown leads to impaired visual function and mislocalization of the photopigment green cone opsin. Moreover, BBS3L RNA, but not BBS3 RNA, is sufficient to rescue both the vision defect as well as green opsin localization in the zebrafish retina. In order to demonstrate a role for Bbs3L function in the mammalian eye, we generated a Bbs3L-null mouse that presents with disruption of the normal photoreceptor architecture. Bbs3L-null mice lack key features of previously published Bbs-null mice, including obesity. These data demonstrate that the BBS3L transcript is required for proper retinal function and organization.

巴德-毕德氏综合征（Bardet-Biedl Syndrome, BBS）是一类异质性综合征型视网膜变性疾病。本研究在已知BBS基因BBS3（ARL6）中发现一种包含额外外显子的全新转录本BBS3L。该转录本具有进化保守性，且主要在眼部组织中表达，提示其在视觉功能中发挥特异性作用。此前我们利用反义寡核苷酸敲低（antisense oligonucleotide knockdown）技术在斑马鱼模型中证实，敲低bbs3会引发斑马鱼BBS的核心表型，包括有纤毛的库普弗囊泡（Kupffer's Vesicle）发育异常以及黑素体逆向运输延迟。与bbs3不同，敲低bbs3L并不会导致库普弗囊泡或黑素体运输异常，反而会造成视觉功能受损与绿视锥视蛋白定位紊乱。进一步实验显示，仅BBS3L核糖核酸（RNA）即可挽救斑马鱼视网膜中的视觉缺陷及绿视蛋白定位异常，而BBS3 RNA则无此挽救效果。为验证Bbs3L在哺乳动物眼部中的功能，我们构建了Bbs3L基因敲除小鼠，该模型出现正常光感受器结构的破坏。与此前已发表的Bbs全基因敲除小鼠不同，Bbs3L敲除小鼠未表现出肥胖等典型BBS特征。上述实验数据证实，BBS3L转录本对于维持视网膜的正常功能与结构完整性至关重要。