Mutations in Complementation Group <i>3R4</i> Disrupt Synaptic Structures in Photoreceptor Terminals

<div><p>(A–F) External morphology of the homozygous eyes of <i>3R4¹</i> (C) and <i>3R4²</i> (E) are normal when compared to those of isogenized control (A). Red eye color marks heterozygous patches. (B), (D), and (F) ERG recordings of control and mutant eyes. Note the reduced depolarization and on/off response. Bar above trace in (B) indicates duration of light stimulus.</p> <p>(G–I) TEM micrographs of lamina cartridges containing control, <i>3R4¹,</i> and <i>3R4²</i> mutant terminals, respectively. Demarcating glia are colored blue and photoreceptor terminals green to accentuate the structures. Note the photoreceptor terminals are disorganized in mutants. The yellow boxes in (G) and (I) indicate the regions shown in (J) and (K), respectively. The red boxes in (G) and (H) indicate the regions shown in (L) and (M), respectively. Scale bar in (G) for (G–I) indicates 1 μm.</p> <p>(J) and (K) Individual terminals that are boxed in (G) and (I) (yellow boxes). <i>nmnat</i> mutant terminals have amorphic active zone structures (red arrows), aberrant capitate projections (blue arrows), aberrant mitochondria (yellow arrowheads), and abnormal membranes (yellow arrow), as well as an aberrant cytoskeleton (blue arrowheads), which are not observed in wild-type terminals. Scale bar indicates 200 nm.</p> <p>(L) and (M) Individual active zones that are boxed in (G) and (H) (red boxes). Compared to the clearly defined wild-type active zone structure (L), <i>nmnat</i> mutant active zones are amorphic and reduced in size. Both wild-type and mutant T-bars are surrounded by synaptic vesicles. Scale bar indicates 200 nm.</p> <p>(N) and (O) Quantification of synapse number and size. No significant difference was found in the average number of active zones per terminal between control (118 terminals counted), and <i>3R4¹</i> (93 terminals counted) or <i>3R4²</i> (71 terminals counted). Synapse size was measured by the width of T-bar platform profile (L) (insert). The size of T-bars in either <i>3R4¹</i> (23 measured) or <i>3R4²</i> (31 measured) is significantly reduced compared to the control (38 measured). An asterisk (*) indicates <i>p</i> < 0.05.</p>