Wenjun Xiong

Loss of Daylight Vision in Retinal Degeneration: Are Oxidative Stress and Metabolic Dysregulation to Blame?

Retinitis pigmentosa is characterized by loss of night vision, followed by complete blindness. Over 40 genetic loci for retinitis pigmentosa have been identified in humans, primarily affecting photoreceptor structure and function. The availability of excellent animal models allows for a mechanistic characterization of the disease. Metabolic dysregulation and oxidative stress have been found to correlate with the loss of vision, particularly in cones, the type of photoreceptors that mediate daylight and color vision. The evidence that these problems actually cause loss of vision and potential therapeutic approaches targeting them are discussed.

Interactions with the Abelson Tyrosine Kinase Reveal Compartmentalization of Eyes Absent Function between Nucleus and Cytoplasm

Eyes absent (Eya), named for its role in Drosophila eye development but broadly conserved in metazoa, possesses dual functions as a transcriptional coactivator and protein tyrosine phosphatase. Although Eyas transcriptional activity has been extensively characterized, the physiological requirements for its phosphatase activity remain obscure. In this study, we provide insight into Eyas participation in phosphotyrosine-mediated signaling networks by demonstrating cooperative interactions between Eya and the Abelson (Abl) tyrosine kinase during development of the Drosophila larval visual system. Mechanistically, Abl-mediated phosphorylation recruits Eya to the cytoplasm, where in vivo studies reveal a requirement for its phosphatase function. Thus, we propose a model in which, in addition to its role as a transcription factor, Eya functions as a cytoplasmic protein tyrosine phosphatase.

Abelson tyrosine kinase is required for Drosophila photoreceptor morphogenesis and retinal epithelial patterning.

Coordinated differentiation and morphogenesis transform the Drosophila retina from a layer of epithelial cells into a complex three‐dimensional organ. In this study we show that the Abelson (Abl) tyrosine kinase localizes to the dynamically remodeling apical‐junctional membrane domains of the developing photoreceptor cells. Analyses of abl mutant clone phenotypes demonstrate that abl is required for enriched localization of adherens junction and apical polarity complex proteins at photoreceptor–photoreceptor cell junctions and apical membrane domains, respectively, for rhabdomere generation and for spatial organization of ommatidial cells along the apical–basal axis of the epithelium. Loss of abl does not alter expression or localization of Enabled (Ena) nor does heterozygosity for ena dominantly suppress the abl phenotypes, suggesting the downstream effector mechanisms used by Abl in the eye may differ from those used in the embryo. Together our results reveal a prominent role for Abl in coordinating multiple aspects of photoreceptor morphogenesis. Developmental Dynamics 240:1745–1755, 2011.

Distinct Expression Patterns of AAV8 Vectors with Broadly Active Promoters from Subretinal Injections of Neonatal Mouse Eyes at Two Different Ages

The retinal expression patterns were analyzed following the injection of serotype 8 adeno-associated virus (AAV8) vectors that utilize two broadly active and commonly used sets of transcription regulatory sequences. These include the human cytomegalovirus (CMV) immediate early (IE) enhancer/promoter and the hybrid CAG element (also known as CAGGS or CBA) composed of a partial human CMV IE enhancer and the chicken β-actin promoter and intron. Subretinal delivery to postnatal day 0 (P0) or 6 (P6) mouse eyes resulted in efficient labeling of retinal cells, but with very distinct patterns. With P0 delivery, AAV8-CMV-GFP selectively labelled photoreceptors, while AAV8-CAG-GFP efficiently labeled both outer and inner retinal neurons, including photoreceptors, horizontal cells, amacrine cells and retinal ganglion cells. With P6 delivery, both vectors led to efficient labeling of photoreceptors and Müller glia cells, but not of inner retinal neurons. Our results suggest that the cell types that express the genes encoded by subretinally delivered AAV8 vectors are determined by both the timing of the injection and the regulatory sequences.

Retinal Axon Guidance Requires Integration of Eya and the JAK/STAT Pathway into Phosphotyrosine-Based Signaling Circuitries in Drosophila

The transcriptional coactivator and phosphatase eyes absent (Eya) is dynamically compartmentalized between the nucleus and cytoplasm. Although the nuclear transcriptional circuits within which Eya operates have been extensively characterized, understanding of its cytoplasmic functions and interactions remains limited. Our previous work showed that phosphorylation of Drosophila Eya by the Abelson tyrosine kinase can recruit Eya to the cytoplasm and that eya–abelson interactions are required for photoreceptor axons to project to correct layers in the brain. Based on these observations, we postulated that photoreceptor axon targeting might provide a suitable context for identifying the cytoplasmic signaling cascades with which Eya interacts. Using a dose-sensitive eya misexpression background, we performed an RNA interference-based genetic screen to identify suppressors. Included among the top 10 hits were nonreceptor tyrosine kinases and multiple members of the Jak/Stat signaling network (hop, Stat92E, Socs36E, and Socs44A), a pathway not previously implicated in axon targeting. Individual loss-of-function phenotypes combined with analysis of axonal projections in Stat92E null clones confirmed the importance of photoreceptor autonomous Jak/Stat signaling. Experiments in cultured cells detected cytoplasmic complexes between Eya and Hop, Socs36E and Socs44A; the latter interaction required both the Src homology 2 motif in Socs44A and tyrosine phosphorylated Eya, suggesting direct binding and validating the premise of the screen. Taken together, our data provide new insight into the cytoplasmic phosphotyrosine signaling networks that operate during photoreceptor axon guidance and suggest specific points of interaction with Eya.