Distribution of MECP2 in the nuclei of retinal cells. (A) MECP2 is abundant in all retinal neurons: in the ganglion cell layer (GCL), inner nuclear cell layer (INL), in bipolar (BC) and amacrine (AC) cells. The signal is present throughout the whole nucleoplasm but is especially strong in chromocenters. In the ONL of adult mice, MECP2 produces a strong signal in cone photoreceptors (CP) whereas rod photoreceptors (RP) have very weak staining only noticeable in the chromocenters (arrowheads). (B) Restoration of conventional nuclear architecture in rod nuclei by Lbr expression in Lbr-TER mice does not increase MECP2 expression. In Lbr-expressing rods (three such nuclei are marked by empty arrowheads), there are multiple chromocenters adjacent to the nuclear periphery. These chromocenters (arrows) remain weakly MECP2-positive and with the staining intensity comparable to that of chromocenters in inverted nuclei not expressing Lbr. For comparison, bright staining of cone nuclei (empty arrows, left and middle upper panels) is shown. Note that all rods with multiple chromocenters adjacent to the nuclear periphery express Lbr (Solovei et al.); LBR staining is not shown on this panel. (C) In R7E mice, rods de-differentiate, partially restore the conventional architecture of their nuclei, and lose their rod identity. This process is accompanied by increased expression of MECP2 which becomes abundant in chromocenters (three such nuclei are marked by arrowheads) and reaches the same level as in neuroretina (upper panel). For comparison, an unaltered rod nucleus is marked (arrow). (D) Retina of rat (D1) and macaque (D2). Similarly to mice, MECP2 produces a bright signal in the GCL, INL, and cones (arrowheads) but is weak to undetectable in rod cells (arrows). Single confocal sections. Scale bars: (A) 10 μm; (B) 5 μm; (C) overview 25 μm, rods 5 μm; (D) overviews 50 μm, ONLs 10 μm.