Conditional Loss of the Exocyst Component Exoc5 in Retinal Pigment Epithelium (RPE) Results in RPE Dysfunction, Photoreceptor Cell Degeneration, and Decreased Visual Function

Oocyte-specific EXOC5 expression is required for mouse oogenesis and folliculogenesis

EXOC5 is a crucial component of a large multi-subunit tethering complex, the exocyst complex, that is required for fusion of secretory vesicles with the plasma membrane. Exoc5 deleted mice die as early embryos. Therefore, to determine the role of EXOC5 in follicular and oocyte development, it was necessary to produce a conditional knockout (cKO), Zp3-Exoc5-cKO, in which Exoc5 was deleted only in oocytes. The first wave of folliculogenesis appeared histologically normal and progressed to the antral stage. However, after IVF with normal sperm, oocytes collected from the first wave (superovulated 21-day-old cKO mice) were shown to be developmentally incompetent. Adult follicular waves did not progress beyond the secondary follicle stage where they underwent apoptosis. Female cKO mice were infertile. Overall, these data suggest that the first wave of folliculogenesis is less sensitive to oocyte-specific loss of Exoc5, but the resulting gametes have reduced developmental competence. In contrast, subsequent waves of folliculogenesis require oocyte-specific Exoc5 for development past the preantral follicle stage. The Zp3-Exoc5-cKO mouse provides a model for disrupting folliculogenesis that also enables the separation between the first and subsequent waves of folliculogenesis.

TIN2-mediated reduction of mitophagy induces RPE senescence under high glucose

The telomere-associated protein TIN2 localizes to both telomeres and mitochondria. Nevertheless, the impact of TIN2 on retinal pigment epithelial (RPE) cells in diabetic retinopathy (DR) remains unclear. This research aims to examine the role of TIN2 in the senescence of RPE and its potential as a therapeutic target. Western blotting and immunofluorescence staining were utilized to identify TIN2 expression and mitophagy. RT-qPCR was employed to identify senescent associated secretory phenotype (SASP) in ARPE-19 cells infected with TIN2 overexpression. To examine mitochondria and the cellular senescence of RPE, TEM, SA-β-gal staining, and cell cycle analysis were used. The impact of TIN2 was examined using OCT and immunohistochemistry in mice. DHE staining and ZO-1 immunofluorescence were applied to detect RPE oxidative stress and tight junctions. Our research revealed that increased mitochondria-localized TIN2 aggravated the cellular senescence of RPE cells both in vivo and in vitro under hyperglycemia. TIN2 overexpression stimulated the mTOR signaling pathway in ARPE-19 cells and exacerbated the inhibition of mitophagy levels under high glucose, which can be remedied through the mTOR inhibitor, rapamycin. Knockdown of TIN2 significantly reduced senescence and mitochondrial oxidative stress in ARPE-19 cells under high glucose and restored retinal thickness and RPE cell tight junctions in DR mice. Our study indicates that increased mitochondria-localized TIN2 induced cellular senescence in RPE via compromised mitophagy and activated mTOR signaling. These results propose that targeting TIN2 could potentially serve as a therapeutic strategy in the treatment of DR.

Liquid crystal monomers disrupt photoreceptor patterning of zebrafish larvae via thyroid hormone signaling

Liquid crystal monomers (LCMs) are the raw material for liquid crystal displays, and their use is steadily increasing in electronic products. Recently, LCMs have been reported to be novel endocrine disrupting chemicals, however, the mechanisms underlying their potential for thyroid hormone disruption and visual toxicity are not well understood. In this study, six widely used fluorinated LCMs (FLCMs) were selected to determine putative mechanisms underlying FLCM-induced toxicity to the zebrafish thyroid and visual systems. Exposure to FLCMs caused damage to retinal structures and reduced cell density of ganglion cell layer, inner nuclear layer, and photoreceptor layer approximately 12.6-46.1%. Exposure to FLCMs also disrupted thyroid hormone levels and perturbed the hypothalamic-pituitary-thyroid axis by affecting key enzymes and protein in zebrafish larvae. A thyroid hormone-dependent GH3 cell viability assay supported the hypothesis that FLCMs act as thyroid hormone disrupting chemicals. It was also determined that FLCMs containing aliphatic ring structures may have a higher potential for T3 antagonism compared to FLCMs without an aliphatic ring. Molecular docking in silico suggested that FLCMs may affect biological functions of thyroxine binding globulin, membrane receptor integrin, and thyroid receptor beta. Lastly, the visual motor response of zebrafish in red- and green-light was significantly inhibited following exposure to FLCMs. Taken together, we demonstrate that FLCMs can act as thyroid hormone disruptors to induce visual dysfunction in zebrafish via several molecular mechanisms.

Mapping of the extracellular RBP4 ligand binding domain on the RBPR2 receptor for Vitamin A transport

The distribution of dietary vitamin A/all-trans retinol/ROL throughout the body is critical for maintaining retinoid function in peripheral tissues and for retinoid delivery to the eye in the support of visual function. In the circulation, all-trans-retinol bound to the RBP4 protein is transported and sequestered into target tissues for long-term storage. Two membrane receptors that facilitate all-trans retinol uptake from RBP4 have been proposed. While it is well established that the membrane receptor, STRA6, binds to circulatory RBP4 for ROL transport into the eye, the second vitamin A receptor, RBPR2, which is expressed in non-ocular tissues, is less characterized. Based on the structural homology between these two RBP4 receptors, published literature, and from our recent work in Rbpr2 ^-/- deficient mice, we hypothesized that RBPR2 might also have high-binding affinity for RBP4 and this mechanism facilitates ROL transport. Herein, we aimed to elucidate the membrane topology and putative RBP4 binding residues on RBPR2 to understand its physiological function for retinoid homeostasis. Using in silico analysis and site-directed mutagenesis, we identified a potential RBP4 binding domain on RBPR2. We employed an in vitro cell-based system and confirmed that mutations of these residues on RBPR2 affected its binding to exogenous RBP4 and subsequently vitamin A uptake. Using Surface Plasmon Resonance assays, we analyzed both the binding affinities and kinetic parameters of wild-type RBPR2 and individual mutants affecting the RBPR2-RBP4 binding domain with its physiological ligand RBP4. These studies not only revealed a putative RBP4 binding domain on RBPR2 but also provided new structural, biochemical, and critical information on its proposed role in RBP4 binding for ROL transport and retinoid homeostasis.

Zebrafish and inherited photoreceptor disease: Models and insights

Photoreceptor dysfunctions and degenerative diseases are significant causes of vision loss in patients, with few effective treatments available. Targeted interventions to prevent or reverse photoreceptor-related vision loss are not possible without a thorough understanding of the underlying mechanism leading to disease, which is exceedingly difficult to accomplish in the human system. Cone diseases are particularly challenging to model, as some popular genetically modifiable model animals are nocturnal with a rod-dominant visual system and cones that have dissimilarities to human cones. As a result, cone diseases, which affect visual acuity, colour perception, and central vision in patients, are generally poorly understood in terms of pathology and mechanism. Zebrafish (Danio rerio) provide the opportunity to model photoreceptor diseases in a diurnal vertebrate with a cone-rich retina which develops many macular degeneration-like pathologies. Zebrafish undergo external development, allowing early-onset retinal diseases to be detected and studied, and many ophthalmic tools are available for zebrafish visual assessment during development and adulthood. There are numerous zebrafish models of photoreceptor disease, spanning the various types of photoreceptor disease (developmental, rod, cone, and mixed photoreceptor diseases) and genetic/molecular cause. In this review, we explore the features of zebrafish that make them uniquely poised to model cone diseases, summarize the established zebrafish models of inherited photoreceptor disease, and discuss how disease in these models compares to the human presentation, where applicable. Further, we highlight the contributions of these zebrafish models to our understanding of photoreceptor biology and disease, and discuss future directions for utilising and investigating these diverse models.