miR-181b expression in the zebrafish brain.
miR-181a and miR-181b belong to the same miRNA family and differ in three nucleotides located outside the seed region. These miRNAs show similar, spatially localised expression in the larval brain. miR-181a and miR-181b expression is quantitatively different from one area to another in the larval brain with strong expression in the retina and brain areas associated with the visual system. miR-181a and miR-181b expression is largely conserved to adult stage although there is downregulation in some areas. Despite overall conservation, we noticed subtle differences in the adult expression of miR-181a and miR-181b that were not obvious at larval stages.
In the larval brain, miR-181a and miR-181b are strongly expressed in retinal ganglion and inner cell layers, migrated pretectal, and tectal cells, that is, cells associated with vision. There are two more areas with strong miR-181a and miR-181b expression, a group of cells located in the central part of the pallium, close to the olfactory bulb and dorsal subpallium, and one in the central medulla oblongata whereas the rest of the larval brain contains weakly expressing cells. Given the adult expression in the olfactory bulb and facial and vagal lobes, the strong telencephalic and medulla oblongata larval expression may correspond to cells that populate these areas. While comparing the regional expression of miR-181a and miR-181b, significant regional differences are not obvious at larval stages (Table C).
Overall, miR-181a and miR-181b conserve their regional expression between larval and adult brain (Table H). For example, miR-181a and miR-181b are expressed in adult pretectal, tectal, hypothalamic and cerebellar cells as it is the case in the larval brain. But there are also differences between larval and adult expression. miR-181a and miR-181b expression is downregulated in the adult thalamus, periventricular posterior tuberculum and tegmentum. In addition, quantitative differences of expression observed in the larval brain are absent from the adult brain.
Finally, comparison of adult miR-181a and miR-181b expression reveals similar regional expression but differences at the cellular level within some areas. Good examples are the caudal hypothalamus and the facial lobe. Both miRNAs are expressed in these areas but likely in different cells. In other areas, like pretectum and tectum, telencephalon and olfactory bulb, they are likely expressed in the same cell types. A. transverse section through the larval telencephalon showing miR-181b expressing cells throughout the ventral (Sv) and dorsal (Sd) subpallium, pallium (P) and olfactory bulb (OB). The asterisk marks the strongly expressing miR-181b cell group in the central pallium, close to the OB and Sd.
B. transverse section through the larval telencephalon (caudal to section A) showing miR-181b expressing cells throughout the ventral (Sv) and dorsal (Sd) subpallium, pallium (P) and olfactory bulb (OB). The asterisk marks the strongly expressing miR-181b cell group in the central pallium, close to the OB and Sd.
C. transverse section through the larval caudal telencephalon at the level of the anterior commissure (ac) showing miR-181b expressing cells in the preoptic area (Po), dorsal subpallium (Sd), pallium (P) and habenula (Ha).
D. transverse section through the larval diencephalon and optic tectum (TeO) showing miR-181b expressing cells in the TeO, preoptic area (Po), eminentia thalami (ET), ventral (VT) and dorsal (DT) thalamus and migrated pretectal area (M1).
E. transverse section through the larval hypothalamus and midbrain showing strongly expressing miR-181b cells in the optic tectum (TeO, periventricular gray zone-pgz and proliferative zone-m).
F. transverse section through the larval hindbrain at the level of the otic capsule (ot) showing miR-181b expressing cells in the medulla oblongata (MO). The asterisk marks the strongly miR-181b expressing cells in the central medulla oblongata likely in proximity to the octaval area (OA?).
G. transverse section through the adult caudal hindbrain showing miR-181b expressing cells in the facial lobe (LVII) and caudal octavolateral nucleus (CON).
H. transverse section through the larval retina showing miR-181b expressing cells in the inner part (amacrine cells) of the inner nuclear layer (INL) and the ganglion cell layer (GCL).
I. transverse section through the adult dorsal diencephalon and rostral optic tectum showing miR-181b expressing cells in the tectal superficial gray (sgz) and central (cz), zones, parvocellular (PSp), magnocellular (PSm) and central (CPN) superficial pretectal nuclei.
J. transverse section through the adult optic tectum showing miR-181b expressing cells in the superficial gray (sgz), central (cz) and periventricular gray (pgz) zones.
K. transverse section through the adult hypothalamus showing miR-181b expressing cells (arrowheads) in the caudal part of the dorsal zone of the periventricular hypothalamus around the lateral ventricular recess (Hd-lr) and mamillary body (CM).
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