Dissertation
Photoreceptor Function and Age-Dependent Rescue Efficacy of Retinal Degeneration in Bardet-Biedl Syndrome
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Spring 2020
DOI: 10.17077/etd.005481
Abstract
Bardet-Biedl syndrome (BBS) is a pleiotropic disorder characterized by obesity, polydactyly, and retinal degeneration, leading to blindness by the second decade of life. The blindness is caused by death of photoreceptor cells. For the purpose of phototransduction, photoreceptor cells in the retina have evolved elaborate cilia, termed the outer segments, and the structure and function of these outer segments greatly differ from that of primary cilia. The BBSome is a multi-protein complex specializing in ciliary trafficking, and it consists of BBS1, BBS2, BBS4, BBS5, BBS7, BBS8, BBS9, and BBIP1. In this thesis, we examine the role of the BBSome in the formation and maintenance of the photoreceptor outer segment, and the efficacy of gene replacement therapies when BBSome function is restored in photoreceptors at different stages of degeneration.
First, we found that the principles for BBSome recruitment to cilia differ between primary cilia and photoreceptor cilia, supporting the existence of tissue-specific mechanisms. Then, we showed that unlike primary cilia, whose formation do not require the BBSome, the proper morphogenesis of photoreceptor outer segments depends on BBSome function. Furthermore, we demonstrated that inactivation of BBSome function in adult mice causes retinal degeneration, suggesting that the BBSome is continuously required for maintenance of the outer segment. Outside of the role of the BBSome in cilia, we found that loss of BBSome function greatly reduces synaptic density between the photoreceptors and the inner neurons, supporting the growing consensus that the role of the BBSome is not limited to cilia. In addition to reduced number of synapses, BBS mutant mice also have misplaced synaptic connections within the photoreceptor layer, outside of their usual positions in the outer plexiform layer. These misplaced synaptic connections in BBS mutant mice are associated with intruding horizontal cells processes. The absence of this phenotype in mice up to three-months of age with photoreceptor-specific deletion of Bbs8, which have significant retinal degeneration, shows that the aberrant extension of horizontal cell processes in the retina is not merely a consequence to retinal degeneration. Instead, this evidence supports a cell-autonomous role of the BBSome in horizontal cells, in addition to a role in photoreceptor cells, which are typically the major target for gene therapy treatment of blindness. Lastly, we studied age-dependent rescue of retinal degeneration in a mouse line in which Bbs8 expression, and by extension BBSome function, can be activated by a removable gene trap to examine the effectiveness of gene replacement therapies. The restoration of BBSome function during earlier stages of retinal degeneration in BBS fully ameliorates retinal phenotypes, permitting the regeneration of outer segments with near normal structure. Restoration of BBSome function at later stages, however, improves the functions of rescued photoreceptors, but these photoreceptors regenerate poorly shaped outer segments and die over time. We identified the regeneration of normal outer segments to be a predictor to long-term rescue success. However, the ability for the photoreceptors to be rescued declines as retinal degeneration progresses, rendering the photoreceptors in late-stage degeneration un-rescuable.
Together, this work illustrates that the BBSome functions in the outer segment morphogenesis pathway, establishes the reliance of photoreceptor survival on their elaborate outer segments, and demonstrates that effective gene therapy is confined to a limited time window during which photoreceptors can fully recover outer segment structure and function.
Details
- Title: Subtitle
- Photoreceptor Function and Age-Dependent Rescue Efficacy of Retinal Degeneration in Bardet-Biedl Syndrome
- Creators
- Ying Hsu
- Contributors
- Val C Sheffield (Advisor)Robert Cornell (Committee Member)Gloria Lee (Committee Member)Seongjin Seo (Committee Member)Budd Tucker (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Molecular and Cellular Biology
- Date degree season
- Spring 2020
- DOI
- 10.17077/etd.005481
- Publisher
- University of Iowa
- Number of pages
- xviii, 179 pages
- Copyright
- Copyright 2020 Ying Hsu
- Language
- English
- Description illustrations
- color illustrations
- Description bibliographic
- Includes bibliographical references (pages 171-179).
- Public Abstract (ETD)
Bardet-Biedl syndrome (BBS) is a rare disease that leads to obesity, extra fingers, and blindness. BBS is also associated with type 2 diabetes and high blood pressure. The blindness is caused by the death of cells in eyes that are responsible for sensing light, the cells responsible for vision. These cells, called photoreceptors, possess large, antenna-like structures for the purpose of sensitive light detection. The normal functions of these antennas require a transport complex called the BBSome complex, which transport cargoes serving the route between the cell and its giant, light sensitive antenna. In patients with BBS, defects in this transport complex, caused by changes in their DNA sequences, or genetic mutations, interrupt the transport route between the cell and its light sensitive antenna. This causes cargo transport to break down. In my thesis, we study what happens to these antennas when they lack this transport complex, how this causes the death of these light-sensing photoreceptor cells, and whether these dying photoreceptor cells can be saved by re-introducing the transport complex. We found that this transport complex is required for the proper formation of light sensitive antennas in photoreceptor cells, and the failure to establish the proper shape of these antennas by the cells leads to cell death. We found that the survival of these light-sensing photoreceptor cells in the eye requires the BBSome transport complex. We then study whether the poorly formed shape of the antennas can be fixed, and the route of transport between the cell and its antenna can be re-established if the transport complex is re-introduced. We found that photoreceptor cell death, and blindness, can be prevented when the function of these transport complexes is re-introduced early enough in the disease. Photoreceptor cells can re-form their antennas that were initially poorly formed if treated at early stages of the disease. At later stages, they cannot do so. Through this work, we define the time window during which photoreceptor cells can be successfully saved for the purpose of designing effective new therapies. This work was performed for optimization of therapies to treat blindness in BBS patients.
- Academic Unit
- Interdisciplinary Graduate Program in Molecular Medicine
- Record Identifier
- 9983968393502771
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