The zebrafish, Danio rerio, has been established as an excellent vertebrate model for the study of developmental biology and gene function. It also has proven to be a valuable model to study human diseases. Here, we reviewed recent publications using zebrafish to study the pathology of human neurodegenerative diseases including Parkinson’s, Huntington’s, and Alzheimer’s. These studies indicate that zebrafish genes and their human homologues have conserved functions with respect to the etiology of neurodegenerative diseases. The characteristics of the zebrafish and the experimental approaches to which it is amenable make this species a useful complement to other animal models for the study of pathologic mechanisms of neurodegenerative diseases and for the screening of compounds with therapeutic potential.

During vertebrate egg maturation, cytokinesis initiates after one pole of the bipolar metaphase I spindle attaches to the oocyte cortex, resulting in the formation of a polar body and the mature egg. It is not known what signal couples the spindle pole positioning to polar body formation. We approached this question by drawing an analogy to mitotic exit in budding yeast, as asymmetric spindle attachment to the appropriate cortical region is the common regulatory cue. In budding yeast, the small G protein Cdc42 plays an important role in mitotic exit following the spindle pole attachment . We show here that inhibition of Cdc42 activation blocks polar body formation. The oocytes initiate anaphase but fail to properly form and direct a contractile ring. Endogenous Cdc42 is activated at the spindle pole-cortical contact site immediately prior to polar body formation. The cortical Cdc42 activity zone, which directly overlays the spindle pole, is circumscribed by a cortical RhoA activity zone; the latter defines the cytokinetic contractile furrow . As the RhoA ring contracts during cytokinesis, the Cdc42 zone expands, maintaining its complementary relationship with the RhoA ring. Cdc42 signaling may thus be an evolutionarily conserved mechanism that couples spindle positioning to asymmetric cytokinesis.

Our findings highlight the importance of comprehensive clinical phenotyping of family members to ultimately provide accurate genetic counseling.

We have generated a line of transgenic zebrafish, Tg(dat:EGFP), in which the green fluorescent protein (GFP) is expressed under the control of cis-regulatory elements of the dopamine transporter (dat) gene. In Tg(dat:EGFP) fish, dopamine (DA) neurons are labeled with GFP, including those in ventral diencephalon (vDC) clusters, amacrine cells in the retina, in the olfactory bulb, in the pretectum, and in the caudal hypothalamus. In the vDC, DA neurons of groups 2-6 are correctly labeled with GFP, based on colocalization analyses. MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) treatments induced a modest but significant loss of DA neurons in groups 2-6 of the vDC. This transgenic line will be useful for the study of DA neuron development and in models of DA neuron loss.

Mutations in the human PTEN-induced kinase 1 (PINK1) gene are linked to recessive familial Parkinson's disease. Animal models of altered PINK1 function vary greatly in their phenotypic characteristics. Drosophila pink1 mutants exhibit mild dopaminergic neuron degeneration and locomotion defects. Such defects are not observed in mice with targeted null mutations in pink1, although these mice exhibit impaired dopamine release and synaptic plasticity. Here, we report that in zebrafish, morpholino-mediated knockdown of pink1 function did not cause large alterations in the number of dopaminergic neurons in the ventral diencephalon. However, the patterning of these neurons and their projections are perturbed. This is accompanied by locomotor dysfunction, notably impaired response to tactile stimuli and reduced swimming behaviour. All these defects can be rescued by expression of an exogenous pink1 that is not a target of the morpholinos used. These results indicate that normal PINK1 function during development is necessary for the proper positioning of populations of dopaminergic neurons and for the establishment of neuronal circuits in which they are implicated.

Autophagy is an evolutionarily conserved lysosomal degradation pathway that plays important roles in cell maintenance, expansion and differentiation. Removal of genes essential for autophagy from embryonic neural stem and precursor cells reduces the survival and inhibits neuronal differentiation of adult-generated neurons. No study has modified autophagy within the adult precursor cells, leaving the cell-autonomous role of autophagy in adult neurogenesis unknown. Here we demonstrate that autophagic flux exists in the adult dividing progenitor cells and their progeny in the dentate gyrus. To investigate the role of autophagy in adult hippocampal neurogenesis, we genetically deleted Autophagy-related gene 5 (Atg5) that reduced autophagic flux and the survival of the progeny of dividing progenitor cells. This significant reduction in survival of adult-generated neurons is accompanied by a delay in neuronal maturation, including a transient reduction in spine density in the absence of a change in differentiation. The delay in cell maturation and loss of progeny of the Atg5-null cells was not present in mice that lacked the essential pro-apoptotic protein Bax (Bcl-2-associated X protein), suggesting that Atg5-deficient cells die through a Bax-dependent mechanism. In addition, there was a loss of Atg5-null cells following exposure to running, suggesting that Atg5 is required for running-induced increases in neurogenesis. These findings highlight the cell-autonomous requirement of Atg5 in the survival of adult-generated neurons.

No abstract

During vertebrate forebrain formation, Dlx homeobox genes play essential roles in the differentiation, migration and survival of subpallial precursor cells that will later give rise to diverse subtypes of γ-aminobutyric acid (GABA)-expressing neurons, including inhibitory cortical interneurons in mammals. They also participate in the regulation of the Gad genes encoding the enzymes necessary for GABA synthesis. In mice, at least four cis-regulatory elements (CREs) control Dlx expression in the telencephalon and diencephalon: URE2 and I12b in the Dlx1/Dlx2 bigene cluster, and I56i and I56ii in the Dlx5/Dlx6 bigene cluster. However, little is known so far with respect to the function of orthologous dlx genes and their regulatory elements during zebrafish GABAergic neuron development. To investigate whether similar dlx-mediated pathways exist in the early developing zebrafish forebrain, we generated independent lines of transgenic zebrafish carrying two distinct GFP reporter constructs driven by a β-globin minimal promoter: one containing a ∼1.4kb dlx5a/dlx6a intergenic sequence (encompassing I56i and I56ii) and one with a ∼1.1kb fragment containing only the I56i CRE, respectively. The expression patterns of these two transgenes were compared with that obtained with another construct containing the ∼1.4kb dlx5a/dlx6a intergenic sequence and driven by a ∼3.5kb dlx6a 5'-flanking fragment. Our comparative analysis showed that GFP expression of the three transgene is largely overlapping throughout the ventral forebrain. Intriguingly, the dlx6a 5'-flanking fragment has a major impact on transgene expression in the mesencephalic tectum. Furthermore, comparison of transgene expression between the ∼1.4kb and ∼1.1kb intergenic fragments did not show any specific spatial expression conferred by I56ii. Almost all GFP-expressing cells in the transgenic zebrafish are GABA-positive and also express various GABAergic interneuron markers. Together, our data suggest that zebrafish dlx5a/dlx6a intergenic CREs may be involved in a conserved genetic pathway necessary for proper dlx expression during zebrafish GABAergic neuron development.