Cook Lab

  • Color Photoreceptor Subtypes

    Program Summary

    In vertebrates, including humans, rod photoreceptors are used for dim light and motion detection, while cone photoreceptors are important for high acuity and color discrimination.  Some retinal degenerative diseases affect all photoreceptors and cause blindness very early in life (e.g., Leber’s congenital amaurosis), whereas others specifically affect rods (e.g., retinitis pigmentosa) or cones (e.g., cone-rod dystrophy) and don’t manifest themselves until the teens or early 20s.  Flies also have rod- and cone-like photoreceptors, called outer and inner photoreceptors, respectively. 

    Our studies have revealed that many of the same factors associated with retinal degeneration in humans are also important for specifying distinct photoreceptor subtypes.   In addition, we are finding that many of the same genetic relationships between different photoreceptor subtypes are conserved between fly and man.  

    Given these similarities, we have recently established a “rescue” system in the fly that allows us to dissect how mutations in human genes associated with retinal degeneration lead to defects in photoreceptor development and differentiation in the fly eye.  Therefore, we have developed several unique genetic tools that will allow us to better understand how different factors lead to distinct retinal degenerative diseases.  This should help us to develop better diagnostic tools and useful therapies for these otherwise blinding diseases.

 
  • color-visual1(2)

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    Human genes associated with retinal degeneration “rescue” photoreceptor defects in the fly eye.

    color-visual1(2)

    Humans use rod photoreceptors for motion and dim light detection and have red, blue and green cones for discriminating colors and for high acuity vision.  Flies also have rod- and cone-like photoreceptors, and are sensitive to UV, blue and green light (e.g. figure 1A – blue (shown as magenta) and green photoreceptors).  Mutations in a transcription factor Orthodenticle (Otd) lead to disruption in the morphology of photoreceptor cell types in the fly eye and disrupt the expression of the opsin proteins responsible for detecting wavelengths of light (e.g. figure 1B).  Likewise, in human, the Otd-related factor, CRX (cone-rod homeobox), is expressed in all photoreceptors, and its mutation leads to a wide range of retinal degenerative diseases, including retinitis pigmentosa, cone-rod dystrophy, and Leber’s congenital amaurosis. 

    Our lab is interested in understanding how factors such as Otd and CRX, which are expressed in all photoreceptors, can exert distinct functions in different photoreceptor subpopulations.  Indeed, genetic and biochemical screens have helped us identify multiple otd-interaction partners that could help explain the cell specific functions of otd and CRX.  Importantly, we have recently found that human CRX can “rescue” the formation of blue photoreceptors and restore proper morphology to photoreceptors lacking Otd (figure 1C).  This system will allow us to examine how specific disease-causing mutations affect CRX function and test the hypothesis that Otd and CRX share similar transcriptional regulatory processes (David Terrell, in preparation).h acuity vision.  Flies also have rod- and cone-like photoreceptors, and are sensitive to UV, blue and green light (e.g. figure 1A).  Mutations in a transcription factor Orthodenticle (Otd) lead to disruption in the morphology of photoreceptor cell types in the fly eye and disrupt the expression of the opsin proteins responsible for detecting wavelengths of light (e.g. figure 1B).  Similarly, the Otd-related protein in humans, CRX (cone-rod homeobox), is expressed in all photoreceptors, and mutations in this factor are associated with a wide range of retinal degenerative diseases, including retinitis pigmentosa, cone-rod dystrophy, and Leber’s congenital amaurosis. 

    Our lab is interested in understanding how factors such as Otd and CRX, which are expressed in all photoreceptors, can exert distinct functions in different photoreceptor subpopulations.  Indeed, genetic and biochemical screens have helped us identify multiple otd-interaction partners that could help explain the cell specific functions of otd and CRX.  Importantly, we have recently found that human CRX can “rescue” the formation of blue photoreceptors and restore proper morphology to photoreceptors lacking Otd (figure 1C).  This system will allow us to examine how specific disease-causing mutations affect CRX function and test the hypothesis that Otd and CRX share similar transcriptional regulatory processes (David Terrell, in preparation).

  • UV vs blue or green-sensitive photoreceptor fates require cross-repression by Prospero and Senseless.

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    UV vs blue or green-sensitive photoreceptor fates require cross-repression by Prospero and Senseless.

    UV vs blue or green-sensitive photoreceptor fates require cross-repression by Prospero and Senseless.

    Although some retinal degenerative diseases affect specific subpopulations of photoreceptors, many of the known genes associated with these diseases are more widely expressed.  Therefore, to develop better therapies for these diseases, it is important to better understand how the different types of photoreceptors form during development. 

    Using the fly as a model system, we have shown that the decision to become UV-, blue- or green-sensitive cone-like photoreceptors requires the reciprocal functions of two transcription factors, Prospero and Senseless.  Prospero is restricted to UV-sensitive photoreceptors, where it blocks these cells from becoming blue- and green-sensitive photoreceptors. 

    Conversely, Senseless is present in blue- and green-sensitive photoreceptors, where it both promotes the differentiation of these cell types and prevents cells from acquiring UV photoreceptor characteristics.  Interestingly, Pros and its vertebrate ortholog Prox1 are tumor suppressors, whereas the Senseless ortholog, Gfi-1, is a potent oncogene.  Therefore, Pros / Prox1 and Sens / Gfi-1 may not only play opposing roles in specifying distinct photoreceptor cell types, but could also play opposing roles in other cellular processes such as proliferation.   

    Such findings emphasize the utility of studies in the fly eye for modeling a wide range of biology processes. For more information, see Cook et al., 2003; Xie et al., 2007; McDonald et al., 2010; Charlton-Perkins and Cook, 2010 on Publications page.

  • Normal eye versus mutant eye.

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    Normal eye versus mutant eye.

    Other transcription factors are critical for deciding blue- vs. green-sensitive photoreceptor cell fates.

    Previous studies have shown that a bistable loop of repression between two signaling proteins, Warts / Lats and Melted, is responsible for deciding green vs. blue-sensitive photoreceptor fates (Mikeladze-Dvali T et al., Cell 2005).   warts (wts) is expressed in green-sensitive photoreceptors and is necessary for activating the green-sensitive opsin-encoding gene, Rh6, and preventing the expression of the blue-sensitive opsin, Rh5. In contrast, melted (melt) is expressed in blue-sensitive photoreceptors and prevents green-sensitive photoreceptor cell fates by repressing warts

    Using molecular, biochemical, and genetic approaches, we are searching for the downstream transcription factors that regulate blue and green- photoreceptor subtype decisions and investigating how these factors interact with Warts-Melted loop.  Below, we show one example of a transcription factor we have found to be important for activating melt to promote blue-sensitive photoreceptor cell fate (Baotong Xie, in preparation).  Importantly, Melt and Warts are critical regulators of two distinct aspects of cell growth and proliferation, and these pathways are often disrupted during cancer formation and progression.  Thus, our results aimed at understanding neuronal diversity in the fly eye also have important implications in cancer research.