Severity of infantile nystagmus syndrome-like ocular motor phenotype is linked to the extent of the underlying optic nerve projection defect in zebrafish belladonna mutant.

Infantile nystagmus syndrome (INS), formerly known as congenital nystagmus, is an ocular motor disorder in humans characterized by spontaneous eye oscillations (SOs) and, in several cases, reversed optokinetic response (OKR). Its etiology and pathomechanism is largely unknown, but misrouting of the optic nerve has been observed in some patients. Likewise, optic nerve misrouting, a reversed OKR and SOs with INS-like waveforms are observed in zebrafish belladonna (bel) mutants. We aimed to investigate whether and how misrouting of the optic nerve correlates with the ocular motor behaviors in bel larvae. OKR and SOs were quantified and subsequently the optic nerve fibers were stained with fluorescent lipophilic dyes. Eye velocity during OKR was reduced in larvae with few misprojecting optic nerve fibers and reversed in larvae with a substantial fraction of misprojecting fibers. All larvae with reversed OKR also displayed SOs. A stronger reversed OKR correlated with more frequent SOs. Since we did not find a correlation between additional retinal defects and ocular motor behavior, we suggest that axon misrouting is in fact origin of INS in the zebrafish animal model. Depending on the ratio between misprojecting ipsilateral and correctly projecting contralateral fibers, the negative feedback loop normally regulating OKR can turn into a positive loop, resulting in an increase in retinal slip. Our data not only give new insights into the etiology of INS but may also be of interest for studies on how the brain deals with and adapts to conflicting inputs.

The equatorial position of the metaphase plate ensures symmetric cell divisions.

Chromosome alignment in the middle of the bipolar spindle is a hallmark of metazoan cell divisions. When we offset the metaphase plate position by creating an asymmetric centriole distribution on each pole, we find that metaphase plates relocate to the middle of the spindle before anaphase. The spindle assembly checkpoint enables this centering mechanism by providing cells enough time to correct metaphase plate position. The checkpoint responds to unstable kinetochore-microtubule attachments resulting from an imbalance in microtubule stability between the two half-spindles in cells with an asymmetric centriole distribution. Inactivation of the checkpoint prior to metaphase plate centering leads to asymmetric cell divisions and daughter cells of unequal size; in contrast, if the checkpoint is inactivated after the metaphase plate has centered its position, symmetric cell divisions ensue. This indicates that the equatorial position of the metaphase plate is essential for symmetric cell divisions.

miR-181a/b control the assembly of visual circuitry by regulating retinal axon specification and growth.

Connectivity and function of neuronal circuitry require the correct specification and growth of axons and dendrites. Here, we identify the microRNAs miR-181a and miR-181b as key regulators of retinal axon specification and growth. Loss of miR-181a/b in medaka fish (Oryzias latipes) failed to consolidate amacrine cell processes into axons and delayed the growth of retinal ganglion cell (RGC) axons. These alterations were accompanied by defects in visual connectivity and function. We demonstrated that miR-181a/b exert these actions through negative modulation of MAPK/ERK signaling that in turn leads to RhoA reduction and proper neuritogenesis in both amacrine cells and RGCs via local cytoskeletal rearrangement. Our results identify a new pathway for axon specification and growth unraveling a crucial role of miR-181a/b in the proper establishment of visual system connectivity and function.

Individual larvae of the zebrafish mutant belladonna display multiple infantile nystagmus-like waveforms that are influenced by viewing conditions.

PURPOSE: Infantile nystagmus syndrome (INS) is characterized by involuntary eye oscillations that can assume different waveforms. Previous attempts to uncover reasons for the presence of several nystagmus waveforms have not led to a general consensus in the community. Recently, we characterized the belladonna (bel) zebrafish mutant strain, in which INS-like ocular motor abnormalities are caused by misprojection of a variable fraction of optic nerve fibers. Here we studied intrinsic and extrinsic factors influencing the occurrence of different waveforms in bel larvae. METHODS: Eye movements of bel larvae were recorded in the presence of a stationary grating pattern. Waveforms of spontaneous oscillations were grouped in three categories: "pendular," "unidirectional jerk," and "bidirectional jerk," and the occurrences of each category were compared within and between individual larvae. Moreover, the effects of the characteristics of a preceding optokinetic response (OKR), of the field of view, and of the eye orbital position were analyzed. RESULTS: The different waveform categories co-occurred in most individuals. We found waveforms being influenced by the preceding OKR and by the field of view. Moreover, we found different kinds of relationships between orbital position and initiation of a specific waveform, including pendular nystagmus in a more eccentric orbital position, and differences among jerk oscillations regarding the beating direction of the first saccade or waveform amplitude. CONCLUSIONS: Our data suggest that waveform categories in bel larvae do not reflect the severity of the morphological phenotype but rather are influenced by viewing conditions.

Analysis of optokinetic response in zebrafish by computer-based eye tracking.

Large-field movements in the visual surround trigger spontaneous, compensatory eye movements known as optokinetic response (OKR) in all vertebrates. In zebrafish (Danio rerio) the OKR is well developed at 5 days post fertilization and can be used in the laboratory for screening of visual performance following genetic manipulations or pharmaceutical treatments. Several setups for measurement of the zebrafish OKR have been described. All of them are based on the presentation of moving gratings to the larva or to the adult fish. However, they differ in the way of presenting gratings and in the method of analysis. Here, we describe a detailed protocol for our newest software that enables computer-generation of the moving stripes and automatic tracking of eye movement. This protocol makes it possible to quantitatively measure OKR in both larvae and adult fishes in a fast and reliable way.

Comparison of infantile nystagmus syndrome in achiasmatic zebrafish and humans.

Infantile nystagmus syndrome (INS; formerly called congenital nystagmus) is an ocular motor disorder characterized by several typical nystagmus waveforms. To date, restrictions inherent to human research and the absence of a handy animal model have impeded efforts to identify the underlying mechanism of INS. Displaying INS-like spontaneous eye oscillations, achiasmatic zebrafish belladonna (bel) mutants may provide new insights into the mystery of INS. In this study, we demonstrate that these spontaneous eye oscillations match the diagnostic waveforms of INS. As a result, zebrafish bel mutants can be used as an animal model for the study of INS. In zebrafish bel mutants, visual pathway abnormalities may contribute to the spontaneous nystagmus via an inverted signal to the pretectal area. We hypothesized that human INS may also be linked to visual pathway abnormalities (possibly underdiagnosed in INS patients) in a similar way.