Yoshikazu Imanishi - US grants

DESCRIPTION (provided by applicant): Three membrane proteins in the rod photoreceptors, rhodopsin, peripherin/rds and cGMP-gated channels, localize to different sub-membrane compartments of the outer segments. This differential compartmentalization is essential for phototransduction signaling, as well as the morphogenesis and maintenance of the rod outer segments. Since photoreceptor outer segments do not have machinery for protein synthesis, proteins need to be synthesized and transported from the photoreceptor inner segments by an intraflagellar transport mechanism. Deficiencies in such transport have been implicated in a broad spectrum of diseases collectively called "retinal ciliopathy". Details of how photoreceptor outer segment morphogenesis is regulated have been controversial;however increasing evidence appears to support a direct association between intraflagellar transport malfunction and retinal ciliopathy. We have developed a new method to fluorescently label newly synthesized proteins in rod photoreceptors and track their movements via confocal and two-photon microscopy in living Xenopus laevis tadpoles. By using this method, we propose: Aim 1. Elucidate the process of photoreceptor disk membrane morphogenesis.;Aim 2. Dissect the functions of signal sequences located at the C-terminal regions of photoreceptor outer segment specific proteins. In Aim 1, we will study the process of disk morphogenesis by using rhodopsin and peripherin/rds fused to a novel fluorescent protein Dendra2. Dendra2 is a photoactivatable protein which turns from green to red upon irradiation by UV or intense blue light, and is able to label newly synthesized proteins in green. In Aim 2, we will differentiate the trafficking pathways mediated by the C-terminal region of three proteins--rhodopsin, peripherin/rds and cGMP-gated channel [unreadable]- subunit. Toward this goal, we will use various Dendra2 fusion proteins and live tadpole imaging. In another aim, we will elucidate the yet unsolved and controversial role of rhodopsin in the process of outer segment morphogenesis. We propose: Aim 3. Determine the structural role of rhodopsin in the integrity of the outer segment. In this aim, we will use a newly established animal model which expresses melanopsin instead of rhodopsin to form the outer segments. We will then determine if the specific structure of rhodopsin is contributing to the integrity of the outer segments by using rhodopsin-melanopsin chimeras expressed in Xenopus rod photoreceptors. The outer segment is not observed in rhodopsin knockout mice, and rhodopsin mislocalization is observed in a broad spectrum of retinal ciliopathies such as Bardet Biedl Syndrome, Usher Syndrome, and other non-syndromic retinitis pigmentosa. Understanding the process and mechanism of outer segment morphogenesis could reveal novel pathways which involve the products of ciliopathy causative genes. PUBLIC HEALTH RELEVANCE: Light is sensed by a part of the photoreceptor neuron called the "outer segment," which disappears or gets shorter in blinding diseases called "ciliopathies". To better understand the process of ciliopathies, we study in two ways how the outer segment is formed. First, we visualize directly the process of outer segment formation by microscopy. Second, we will determine the structural role of the key protein, rhodopsin, in building the outer segment.

Title: Maintenance of cone photoreceptor outer segments Abstract: Cone photoreceptors are essential for perception of color and for high-acuity vision. In industrialized countries, we rely more on cone mediated vision than on rod mediated vision, both during the day and during most of the night. Degeneration of cones is causative to a number of blinding disorders, including macular degeneration, retinitis pigmentosa, and cone dystrophy. Etiology of those blinding disorders is often associated with deficiencies in the maintenance of the photoreceptive outer segment (OS). The process of OS morphogenesis is poorly understood in cones. We have developed a new method to fluorescently label newly synthesized proteins and track their movements via fluorescence microscopy in live Xenopus laevis cones. By using this method, we will study the mechanism of cone OS protein renewal. Cone OSs contain open disks (also called lamellae) that are interconnected and continuous with the plasma membrane. We will clarify whether diffusion barrier exist between open disks and gain insight into the redistribution of membrane proteins after their delivery to the cone OSs. Such studies are critical for the understanding of how cone OS membrane proteins are renewed. Impaired maintenance of cone OS is observed in a broad spectrum of blinding disorders such as Bardet Biedl Syndrome, Usher Syndrome, and other non-syndromic forms of retinitis pigmentosa. This study could reveal novel pathways that would be harnessed to treat these blinding disorders.

Title: Photoreceptor dysfunction associated with rhodopsin mislocalization Abstract: Rhodopsin mislocalization is observed in various blinding disorders including syndromic and non-syndromic retinitis pigmentosa. In most of these disorders rhodopsin mislocalizes to the inner segment (IS) plasma membrane (PM). Growing evidence suggests that PM mislocalization of rhodopsin is the root cause of photoreceptor degeneration, but how such mislocalization causes rod photoreceptor degeneration remains unknown. In this project, we will test the hypothesis that mislocalized rhodopsin disrupts PM homeostasis thereby causing dysfunction and degeneration of rod photoreceptor neurons. In rod photoreceptors, rhodopsin is synthesized at an extremely high rate and delivered to the base of the outer segments (OSs). This high rate of synthesis is balanced with a high rate of catabolism. Rhodopsin-containing disk membranes are shed at the tip of the OSs, engulfed, and digested by the retinal pigment epithelial (RPE) cells. When rhodopsin mislocalizes, a massive amount of rhodopsin is delivered to the PM of the ISs, where rhodopsin-containing membranes have no apparent contact with RPE cells. Nevertheless, we recently found that mislocalized rhodopsin is actively eliminated from the PM while new rhodopsin molecules are continuously delivered to this structure. The mechanism of elimination will be the subject of this study (Aim 1). Photoreceptor cells are terminally differentiated neurons that survive during the entire lifespan of vertebrates, including humans. Therefore, the contents of photoreceptor cells must be continuously renewed. How this renewal occurs for the OS structure is well-established, but has not been studied for the IS. We will address the renewal mechanism of IS PM proteins in rod photoreceptors and investigate the pathological process of disrupting the IS PM protein homeostasis by massive mistrafficking of rhodopsin there (Aim 2).

Title: Proteostasis modulation in inherited blinding disorders. Abstract: Missense mutations of RPE or photoreceptor specific genes often cause inherited blinding disorders. These mutated genes frequently yield protein products that are highly unstable and prone to proteasomal degradation. We recently invented a novel drug discovery strategy and identified a small molecule which can stabilize these mutated protein products. In this project, we will test the therapeutic hypothesis that proteostasis modulation is a viable and general therapeutic strategy for treating inherited blinding disorders caused by such protein destabilizing missense mutations. Currently, no cures or treatments exist for the majority of these blinding disorders. Toward the goal of fulfilling such unmet medical needs, we will study the specificity (Aim1) and mechanism (Aim2) of the proteostasis modulation by our novel small molecule. Moreover, we will prove the concept of the proteostasis modulation therapy using mouse models of severe visual impairment and blindness (Aim3). This study will reveal novel molecular pathways for stabilizing proteins whose loss of functions are associated with inherited disorders. Such pathways will become attractive targets for various therapeutic molecules.