Advisory Committee Chair
Alecia K Gross
Advisory Committee Members
Marina Gorbatyuk
Thomas T Norton
John M Parant
Elizabeth S Sztul
Bradley K Yoder
Document Type
Dissertation
Date of Award
2018
Degree Name by School
Doctor of Philosophy (PhD) School of Optometry
Abstract
The primary cilium is an evolutionary conserved structure with numerous motility and sensory-related functions, dynamically concentrating signaling molecules to developmental, homeostatic and sensory pathways. Rod and cone photoreceptor cells are highly modified primary cilia, which reside in the back of the eye within the neural retina and are responsible for dim light sensitivity and visual acuity/ color vision, respectfully. These cells require massive protein production and proper protein trafficking on a daily basis. The major protein complexes that govern protein trafficking within photoreceptors form the connecting cilium, are the transition zone which serves as a gatekeeper, regulating protein trafficking into and out of the cilia and the BBSome, which is involved with vesicular protein sorting and retrograde trafficking. Mutations occurring within both of these components of connecting cilium result in retinal degeneration-associated ciliopathies, of which the underlying cause remains elusive. This work investigates proteins residing in compartments of the connecting cilium, Meckel-Grüber Syndrome 6 (Mks6) of the transition zone and Bardet-Biedl Syndrome 5 (BBS5) of the BBSome, identifying their distinct roles in retinal protein trafficking. Due to the embryonic lethality caused by congenital loss of Mks6, a conditional knockout mouse model (Mks6Δ) was assessed at two time points, a juvenile induced time point to examine the role of Mks6 in photoreceptor development and an adult induced time point to assess its role in photoreceptor maintenance. Congenital knockout mouse model of BBS5 (Bbs5-/-) was viable; therefore it was evaluated over several time points of 2, 3 and 9 months. The effects of Mks6 and BBS5 loss on overall retinal histology and amount of photoreceptor cell death was analyzed and measured. Dark- and light-adaptation studies provide the ability to assess differences in retinal protein localization in knockout mouse models compared to their respective age matched controls. Retinal function was assessed through electroretinography, testing for rod and cone photoreceptor function separately. Transmission electron microscopy was performed in order to examine connecting cilium and photoreceptor ultrastructure. In juvenile-induced Mks6Δ mice, a significant amount of cell death was observed and was associated with rhodopsin mislocalization and a severe arrestin-1 mislocalization occurring in light adapted animals exclusively. In adult-induced Mks6Δ mice retinal function was assessed every week within the same cohort of mice and it was found that rod photoreceptor signal declined prior to cone photoreceptor signal, with complete loss of retinal function by 12 weeks post induction. At this time point, only two rows of nuclei remained in the outer nuclear layer with minimal outer segments present. Therefore, Mks6 plays a structural role and is important for outer segment maintenance, more specifically to rod photoreceptor protein trafficking and function. In Bbs5-/- mice retinal histology revealed no significant differences at 2 months but significant amounts of retinal cell death and loss nuclei in the outer nuclear layer at 9 months were assessed. Electroretinography revealed significant decreases in scotopic a-and b-wave amplitudes of rod photoreceptor function and a total loss of cone signal at 2 months. In light and dark-adapted studies it was found that rhodopsin and transducin properly localize, but arrestin-1 was found mislocalized in a light dependent manner only. Cone photoreceptor phototransduction components were analyzed and it was found that Bbs5-/-mice have severe arrestin-4, M-and S-opsin, CNGA3 and cone transducin, GNAT2 mislocalization. Whereas, peripherin-2 which is expressed in both rod and cone disks was properly localized, suggesting that BBS5 has a cargo protein specific role for cone phototransduction components. Transmission electron microscopy showed an increase in abundance of aberrant outer segment disk membranes at 3 months. In all, this work provides insight for the distinct and similar roles these connecting cilium components play within rod and cone photoreceptors, respectively. The data gathered in these studies will extend to a wide range of other organs and cell types, conveying insight of the mechanisms that cause these devastating ciliopathies.
Recommended Citation
Bales, Katie Leigh, "Connecting Cilium Proteins Bbs5 And Mks6 And Their Roles In Retinal Degeneration" (2018). All ETDs from UAB. 1094.
https://digitalcommons.library.uab.edu/etd-collection/1094