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Xiangyun Wei, Jian Zou; Novel Cell-cell Adhesion Mediated By The Extracellular Domains Of Crb2 Is Required For Photoreceptor Patterning And Survival. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5351.
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In many vertebrate species, cone photoreceptors assemble into exquisite two-dimensional arrays, namely photoreceptor mosaics. The mechanisms by which photoreceptor mosaics are established and maintained remain largely unknown. The retina and many other epithelium-derived tissues express the Crumbs (Crb) polarity proteins. While certain mutations in the extracellular domain of human CRB1 cause Retinitis Pigmentosa (RP) or Leber’s Congenital Amaurosis (LCA), the specific functions of the domain and molecular etiology of CRB1-related RP and LCA remain to be understood. This study was undertaken to investigate the roles of Crb polarity proteins in mediating photoreceptor lateral adhesion at the inner segments for photoreceptor development and maintenance in zebrafish.
The authors generated specific antibodies against Crb1, Crb2a, and Crb2b and analyzed Crb retinal expression patterns by immunohistochemistry. To test the adhesion function of Crb, the authors performed pull-down assays, HEK293 cell culture expression and aggregation assays, a rescue assay by expressing wildtype Crb2a in crb2a mutant embryos, and a loss-of-function assay by transgenically overexpressing a dominant negative form of Crb2b in photoreceptors.
Crb2a and Crb2b are differentially expressed in the subapical regions of the photoreceptor layer: Crb2a is expressed broadly in all types of photoreceptors and Muller cells; whereas Crb2b is expressed preferentially in green, red, and blue cones. The crb2b gene is expressed both in a shorter form (Crb2b-sf) and a longer form (Crb2b-lf). The extracellular domains of Crb2a and Crb2b mediate homotypic physical adhesion. Expression of Crbs in HEK293 cells induced cell aggregation. The crb2a mutant photoreceptors resumed adhesive properties and formed aggregates when they transgenically expressed wildtype Crb2a. Antagonization of endogenous wildtype Crb2b function by transgenically expressing a dominant negative form of Crb2b-sf disrupted photoreceptor mosaics and induced progressive degeneration of certain photoreceptors.
The authors provided evidence that differential lateral cell-cell adhesion mediated by the Crb polarity proteins in the photoreceptor layer plays an essential role in the organization of the photoreceptor mosaics and photoreceptor survival. The study sheds light on the pathological pathways of CRB1-related human retinal degenerative conditions, such as RP and LCA. The Crb based cell-cell adhesion may underlie the cellular pattern formation of other epithelium-derived tissues as well.
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