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Small teleost fish including zebrafish and medaka have been used as animal models for research because of their small body size, vast amounts of eggs produced, their rapid development, low husbandry costs, and transparency during embryogenesis. Although the body size and appearance seem different, fish and mammals including human still possess anatomical and functional similarities in their brains. This review summarizes the similarities of brain structures and functions between teleost fish and mammalian brains, focusing on the dopamine system, functional regionalization of the cerebellum, and presence of the nucleus ruber.

BACKGROUND Members of the junctional adhesion molecule (JAM) family function as cell adhesion molecules and cell surface receptors. The zebrafish genome contains six different jam genes, and jam-b and jam-c were shown to be essential for myoblast fusion during skeletal muscle development. However, little is known about jam-b2 expression and function. RESULTS We isolated the cDNA of zebrafish jam-b2. jam-b2 is expressed specifically in extraocular muscles (EOMs), jaw muscles, and pectoral fins in zebrafish larvae, but not in trunk muscles. The identified jam-b2 expression pattern is supported by the analysis of a zebrafish Gal4-enhancer trap line, in which the coding sequence of the transcriptional activator KalTA4 together with a Gal4-dependent UAS-mCherry expression cassette was inserted into the jam-b2 locus. Intercrosses with an UAS:EGFP strain proves the possibility for targeting transgene expression to EOMs, jaw muscles and fins. Finally, we characterized the concerted contraction pattern of EOMs in larvae performing an optokinetic response. CONCLUSIONS The expression pattern of jam-b2 suggests that it may contribute different properties to EOMs, jaw muscles, and pectoral fins. The jam-b2:KalTA4-UAS-mCherry transgenic strain serves a dual role as both a reporter for these muscles and as a valuable genetic tool for targeting transgene expression to EOMs.

Perturbations in protein folding and degradation are key pathological mechanisms in neurodegenerative diseases, including Parkinson's disease (PD). Recent evidence suggests that mishandling of proteins may play an important role in the pathogenesis of PD. We have utilized medaka fish to monitor the effects of injecting neurotoxins into the CSF space. In this study, ammonium chloride, tunicamycin, and lactacystin were tested for their ability to disturb lysosomal proteolysis, N-glycosylation in the endoplasmic reticulum, and proteasomal degradation, respectively. All of the substances tested induced selective loss of dopaminergic neurons, movement disorders and inclusion bodies. Among them, the features of the inclusion bodies that developed after ammonium chloride injection mimicked those of PD: co-localization of ubiquitin and phosphorylated α-synuclein, as well as the presence of LC3 protein in the inclusion bodies. Our study demonstrated that medaka fish are useful for examining the effects of environmental toxins and lysosome inhibition, and lysosome inhibitors may be factors in the development of PD.

Recent findings suggest that a defect in the ubiquitin-proteasome system plays an important role in the pathogenesis of Parkinson's disease (PD). A previous report (McNaught et al. 2004) demonstrated that rats systemically injected with proteasome inhibitors exhibited PD-like clinical symptoms and pathology. However, because these findings have not been consistently replicated, this model is not commonly used to study PD. We used medaka fish to test the effect of systemic administration of proteasome inhibitors because of the high level of accessibility of the cerebrospinal fluid in fish. We injected lactacystin or epoxomicin into the CSF of medaka. With proteasome inhibition in the medaka brain, selective dopaminergic and noradrenergic cell loss was observed. Furthermore, treated fish exhibited reduced spontaneous movement. Treatment with proteasome inhibitors also induced the formation of inclusion bodies resembling Lewy bodies, which are characteristic of PD. Treatment with 6-OHDA also induced dopaminergic cell loss but did not produce inclusion bodies. These findings in medaka are consistent with previous results reporting that non-selective proteasome inhibition replicates the cardinal features of PD: locomotor dysfunction, selective dopaminergic cell loss, and inclusion body formation.