视神经损伤

    A Drosophila perspective on retina functions and dysfunctions
  • Figure 1|Basic structures underlying the early stages of visual processing of adult D. Melanogaster and experimental procedure for phototaxis assay.

    A brief comparative view of Drosophila eye: The Drosophila compound eye is a complex structure of about 750–800 functional units known as ommatidia (Paulk et al., 2013) (Figure 1A). The cornea and the crystalline cone occupy the most distal part of each ommatidium, which together constitute the dioptric apparatus. Ommatidia form the retina and are made up of eight photoreceptor cells, R1–R8, that are highly polarized epithelial cells arranged in an hexagonal pattern, and eleven accessory cells. R1–R6 cells are located in the periphery of the ommatidium and are the outer photoreceptors. They surround the inner photoreceptors R7 and R8, which occupy the distal and the proximal portion of the center of the ommatidium, respectively. All photoreceptors have a stalk and a rhabdomere. Rhabdomere is the specialized visual organelle composed of thousands of microvilli and supported by an actin cytoskeleton (Figure 1B). Of notice, R7 and R8 share a common rhabdomere R7,8 (R7 distal, R8 proximal). 
    The visual systems of Drosophila and vertebrate share structural, developmental and functional features (Paulk et al., 2013). Both in vertebrate and fly retina there are specific neuronal cell types, which are arranged in parallel layers (Figure 1C). In vertebrates, cell bodies are distributed into three nuclear layers, that are the outer nuclear layer with photoreceptors, the inner nuclear layer that contains the interneurons (horizontal, bipolar and amacrine cells), and the ganglion cell layer, with retina ganglion cells and some amacrine cell bodies. The synapses of these cells form the outer and the inner plexiform layers. In flies, synapses are present in neuronal layers other than retina sensu stricto, since photoreceptors detect light and then transmit the signal to the optic lobe. The lobe comprises four layers of retinotopically-organized neuropiles called lamina, medulla, lobula and lobula plate (lobula and lobula plate constituting the lobula complex). Functionally, the outer Drosophila photoreceptors R1–R6 detect achromatic contrasts and mediate motion vision, similarly to vertebrate rods, while R7 and R8 are used primarily to detect color and polarization, being thus equivalent to vertebrate cones (Paulk et al., 2013). The visual information is processed in the optic lobe. In particular, photoreceptors synapse indirectly or directly in different layers of the medulla. The axons of R1–R6 cells project to the lamina in a single cartridge. In terms of network, the fly lamina is similar to the outer plexiform layer of vertebrate retina (Paulk et al., 2013).Neurons from a single cartridge in the lamina synapse to specific layers within a medulla column. R7–R8 neurons arborize directly in the medulla in such a way that the motion and color vision remain separate. Thus, the medulla elaborate a wide variety of visual information, such as colors, mobility detection and shape assessment, with an high degree of compartmentalization, like the vision system of vertebrates (Paulk et al., 2013).

    Insights on Drosophila retina in health and disease: In a general point of view, vision represents a source of information on the environment in which animals live, including those required to recall food or nest. It has been demonstrated that D. melanogaster is capable of visual place learning and remembers the location of objects, being thus an useful system to study complex behaviors such as spatial memory (Ofstad et al., 2011). 
    Fly eye was extensively studied at functional level by electroretinogram recordings that measure electrical activities of photoreceptors, second order neurons and adjacent cells during first steps of vision process. However, an indirect approach could be also adopted to assess phototransduction performance. Indeed, Drosophila shows strong phototactic response since is attracted by light, avoiding shaded areas, when given a choice (Paulk et al., 2013). In this respect, phototaxis, in the absence of altered mobility and activity, is a robust and quick assay to reveal decreased fly responsiveness to the light because of vision defects. For instance, the phototactic responses of adult D. melanogaster can be measured easily utilizing a glass and transparent tube placed horizontal and perpendicular to a light source, in a dark room (Catalani et al., 2021a, b). During the test (light on) a camera records fly behavior and their movement (horizontal walking) towards the light source (Figure 1D). Animals are then analyzed at specific intervals in each part of the tube. Individuals that have vision defects do not reach the source as fast as the control, are not attracted by the light, are motionless, move perpendicular to the light or unbiased towards and away from the light. To note, phototaxis is helpful also to uncover neural function and networks such as visual perception since the assay may be related to the time of the day and the internal state of the flies. In addition, phototaxis may be used as an indicator of visual ability of flies. Indeed, phototactic behavior involves both chromatic and achromatic discrimination, it is not merely related to the intensity of light but also depends on the wavelength (Paulk et al., 2013). Taking advantages from the susceptibility to specific wavelengths of light, a programmable optical stimulator has been developed to induce retina degeneration in Drosophila, by exposing photoreceptors to blue but not red light (Chen et al., 2017). 


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  • 发布日期: 2021-10-11  浏览: 593
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