Lego Self-Assembly Model of Zebrafish RGB Cone Pentamer Formation: Unique Role of Crumbs2b in Cone Coalescence via Planar Orientational Cell Adhesions.

Purpose: Vertebrate retinal photoreceptors are non-randomly patterned into planar mosaics. In zebrafish, red, green, and blue (RGB) cones coalesce into linear mirror-symmetric pentamers (G-R-B-R-G) by unknown mechanisms. Here, we investigate the role of the Crumbs2b (Crb2b) polarity protein in pentamer formation and maintenance. Methods: The crb2bsa21178 null mutation was used to study the Crb2b functions. Confocal microscopy was used to examine tissue morphologies and protein distribution. Results: Crb2b, which localizes anisotropically in the planar and apicobasal axes in RGB cones, is required for RGB cone pentamer formation. Crb2b deficiency causes RGB cones to form a new bead-thread configuration, a finding that reveals additional new insights: Crb2b is required for parallel alignment of RGB cone interfaces; the pentamer patterning defect induced by loss of Crb2b is accompanied by planar patterning defect in rods, ultraviolet cones, and Müller cell apical processes; among RGB cones, G-R and R-B are preferred coalescences, which do not require Crb2b; RGB cones display self-avoidance, in which Crb2b does not play a major role; cone survival does not require Crb2b; and, without affecting the overall retinal apicobasal polarization, Crb2b deficiency causes excessive rods and thickening of the inner plexiform layer. Conclusions: We propose a Lego self-assembly model as a theory to explain the formation and maintenance of RGB cone pentamers. This model has six elements, among which Crb2b may regulate the geographical distribution and strengths of planar orientational cell adhesions (pOCA). This model exemplifies how distinct OCAs can promote self-assembly of various tissue architectures.