The Formation and Renewal of Photoreceptor Outer Segments.

The visual system is essential for humans to perceive the environment. In the retina, rod and cone photoreceptor neurons are the initial sites where vision forms. The apical region of both cone and rod photoreceptors contains a light-sensing organelle known as the outer segment (OS), which houses tens of thousands of light-sensitive opsins. The OSs of photoreceptors are not static; they require rhythmic renewal to maintain normal physiological functions. Disruptions in OS renewal can lead to various genetic disorders, such as retinitis pigmentosa (RP). Understanding the patterns and molecular mechanisms of photoreceptor OS renewal remains one of the most intriguing topics in visual biology. This review aims to elucidate the structure of photoreceptor OSs, the molecular mechanisms underlying photoreceptor OS renewal, and the retinal diseases resulting from defects in this renewal process. Additionally, we will explore retinal diseases related to photoreceptor OS renewal and potential therapeutic strategies, concluding with a discussion on future research directions for OS renewal.

Loss of Ift74 Leads to Slow Photoreceptor Degeneration and Ciliogenesis Defects in Zebrafish.

Cilia are microtubule-based structures projecting from the cell surface that perform diverse biological functions. Ciliary defects can cause a wide range of genetic disorders known collectively as ciliopathies. Intraflagellar transport (IFT) proteins are essential for the assembly and maintenance of cilia by transporting proteins along the axoneme. Here, we report a lack of Ift74, a core IFT-B protein, leading to ciliogenesis defects in multiple organs during early zebrafish development. Unlike rapid photoreceptor cell death in other ift-b mutants, the photoreceptors of ift74 mutants exhibited a slow degeneration process. Further experiments demonstrated that the connecting cilia of ift74 mutants were initially formed but failed to maintain, which resulted in slow opsin transport efficiency and eventually led to photoreceptor cell death. We also showed that the large amount of maternal ift74 transcripts deposited in zebrafish eggs account for the main reason of slow photoreceptor degeneration in the mutants. Together, our data suggested Ift74 is critical for ciliogenesis and that Ift proteins play variable roles in different types of cilia during early zebrafish development. To our knowledge, this is the first study to show ift-b mutant that displays slow photoreceptor degeneration in zebrafish.

Novel and rapid osteoporosis model established in zebrafish using high iron stress.

Osteoporosis is a global public health concern and, it can result from numerous pathogenic mechanisms, many of which are closely related with age, nutritional disorders, endocrine imbalance, or adverse drug side effects presented by glucocorticoids, heparin, and anti-epileptics. Given its wide range etiologies, it is crucial to establish an animal model of osteoporosis for use in screening potential drugs quickly and effectively. Previous research has reported that an accumulation of elevated iron in the body is an independent risk factor for osteoporosis. As such, we sought to use both zebrafish larvae and adults to model an osteoporosis phenotype using high iron stress (FAC, ferric ammonium citrate). Skeletal staining results suggested that iron-overload caused a significant decrease in bone calcification as well as severe developmental cartilage defects. In addition, osteoblast and cartilage-specific mRNA expression levels were downregulated after exposure to a high-iron environment. Most importantly, we demonstrated in both larval and adult fish that high iron-induced osteogenic defects were significantly rescued using alendronate (AL), a drug known to be effective against to human osteoporosis. Even more, the repair effect of AL was achieved by facilitating osteoblast differentiation and targeting Bmp signaling. Taken together, our findings propose an rapid and effective osteoporosis model, which could be used widely for future osteoporosis drug screening.

Coordinate involvement of Nodal-dependent inhibition and Wnt-dependent activation in the maintenance of organizer-specific bmp2b in zebrafish.

A vertebrate signaling center, known in zebrafish as the organizer, is essential for axis patterning and formation and is regulated by multiple cell signaling pathways, including Wnt, Nodal, and Bmp. Organizer-specific Bmp2b plays important roles in the maintenance of the Bmp activity gradient and dorsal-ventral patterning. However, it is unknown how transcription of bmp2b in the organizer is regulated. In this study, we generated a bmp2b transgenic line Tsg(-2.272bmp2b:gfp) that reproduced organizer-specific bmp2b expression. Dissection analysis revealed that a 0.273-kb minimal promoter was indispensable for bmp2b expression in the dorsal organizer. Reporter assays showed that organizer-specific bmp2b is negatively regulated by the Nodal signal and positively regulated by the Wnt signal in both embryos and cell lines. Promoter analysis and chromatin-immunoprecipitation (ChIP) indicated that one consensus Smad-binding element (SBE) (CAGAC) and one Lef/Tcf-binding element (LBE) (AGATAA) were present in the 0.273-kb promoter, and could be directly bound by Smad2 and ß-catenin proteins. Together, these results suggest that maintenance of organizer-specific bmp2b expression involves opposite and concerted regulation by Nodal and Wnt signaling.