Molecular Heterogeneity among Primary Motoneurons and within Myotomes Revealed by the Differential mRNA Expression of Novel Islet-1 Homologs in Embryonic Zebrafish

Abstract

Zebrafish embryos have three or four identifiable primary motoneurons per hemisegment. We previously reported that, while several ventral cells initially express the zebrafish Islet-1 (Isl-1) gene, a member of the LIM/homeobox gene family, the expression of this gene becomes restricted to a single or a pair of cells slightly anterior to each segment border by 16 hr after fertilization. Double staining by in situ hybridization and immunohistochemistry strongly suggested that these cells were mainly rostral primary motoneurons. Here, we have isolated two novel zebrafish cDNA clones for more Isl-1 family genes, termed zfIsl-2 and zfIsl-3. zfIsl-2 mRNA starts to be expressed in the ventral midsegmental cells per hemisegment around 15 hr. Double labeling experiments have shown that these midsegmental cells are the caudal primary motoneuron (CaP) and its variant equivalence pair. Our results revealed the heterogeneity in the expressed genes among primary motoneurons before the fates of the primary motoneurons are irreversibly determined, and further suggest the involvement of the Isl-1 and zfIsl-2 genes in the determination of cellular identities by primary motoneurons in embryonic zebrafish. zfIsl-3 mRNA is not expressed in motoneurons but is expressed at 17 hr, mainly in the ventral myotomes. This suggests that zfIsl-3 may be involved in the regional specification of the myotome and also in target recognition by CaP. zfIsl-2 is also expressed throughout the developing eye and tectal region of the midbrain, the target for the retinal axons. In the ventral spinal cord of the spadetail mutant embryo, which has defects in the somites, the cells expressing zfIsl-2 mRNA significantly decreased in number in contrast to the increase in cells expressing Isl-1 mRNA, suggesting the influence of the somites on the expression of both genes.

Neuroepithelial cells require fucosylated glycans to guide the migration of vagus motor neuron progenitors in the developing zebrafish hindbrain

“…9A-C). Because Notch signaling has an antineurogenic effect on neural stem cells, we observed primary motor neurons and Rohan-Beard sensory neurons in the spinal cord (Inoue et al, 1994;Tokumoto et al, 1995). There were no significant differences detected in the spinal cord between wild-type and twd rw685 mutant embryos ( Fig.…”

Expression ofzTlxA, aHox11‐like gene, in early differentiating embryonic neurons and cranial sensory ganglia of the zebrafish embryo

“…A detailed study (Sipple, 1998) of the timing of arborization of the dendrites and expression of zTlxA by Rohon-Beard cells indicates that the lessened expression at particular anteroposterior levels is correlated with cessation of dendritic growth cone extension. Comparison of expression of zTlxA with islet-1 and islet-2, two other genes known to be expressed in developing Rohon-Beard neurons (Inoue et al, 1994;Tokumoto et al, 1995), revealed that expression of zTlxA and islet-1 is initiated simultaneously at 11 hpf in these cells, whereas islet-2 expression is initiated approximately 5 hr later (data not shown).…”

Islet1 selectively promotes peripheral axon outgrowth in Rohon‐Beard primary sensory neurons

“…In zebrafish, the members of the Isl family (Isl1, Isl2a, and Isl2b, which were previously known as Isl1, Isl2, and Isl3, respectively) are expressed not only in motor neurons but also in Rohon-Beard (RB) primary sensory neurons (Appel et al, 1995;Inoue et al, 1994;Miyashita et al, 2004;Segawa et al, 2001;Tokumoto et al, 1995;Uemura et al, 2005). RB neurons are multipolar neurons that extend their central axons longitudinally within the dorsal spinal cord both rostrally and caudally, and also extend their highly branched peripheral axons subcutaneously, transmitting sensory stimuli to the CNS (Bernhardt et al, 1990;Ribera and Nusslein-Volhard, 1998).…”