Abstract
Signals issued by dorsal roof and ventral floor plates, respectively, underlie the major patterning process of dorsalization and ventralization during vertebrate neural tube development. The ventrally produced morphogen Sonic hedgehog (SHH) is crucial for vertebrate hindbrain and spinal motor neuron development. One diagnostic gene for motor neurons is the LIM/homeodomain gene islet1, which has additional ventral expression domains extending into mid- and forebrain. In order to corroborate motor neuron development and, in particular, to improve on the identification of poorly documented zebrafish forebrain islet1 populations, we studied adult brains of transgenic islet1-GFP zebrafish (3 and 6 months). This molecular neuroanatomical analysis was supported by immunostaining these brains for tyrosine hydroxylase (TH) or choline acetyltransferase (ChAT), respectively, revealing zebrafish catecholaminergic and cholinergic neurons. The present analysis of ChAT and islet1-GFP label confirms ongoing adult expression of islet1 in zebrafish (basal plate) midbrain, hindbrain, and spinal motor neurons. In contrast, non-motor cholinergic systems lack islet1 expression. Additional presumed basal plate islet1 positive systems are described in detail, aided by TH staining which is particularly informative in the diencephalon. Finally, alar plate zebrafish forebrain systems with islet1 expression are described (i.e., thalamus, preoptic region, and subpallium). We conclude that adult zebrafish continue to express islet1 in the same brain systems as in the larva. Further, pending functional confirmation we hypothesize that the larval expression of sonic hedgehog (shh) might causally underlie much of adult islet1 expression because it explains findings beyond ventrally located systems, for example regarding shh expression in the zona limitans intrathalamica and correlated islet1-GFP expression in the thalamus.
Highlights
IntroductionA major patterning process in the developing vertebrate neural tube (i.e., central nervous system, CNS) is the generation of gradients of morphogens both from dorsally (roof plate) and ventrally (floor plate and prechordal plate) which eventually results in dorsalization and ventralization of neural cell fates (Martí and Bovolenta, 2002; Gilbert, 2014)
A major patterning process in the developing vertebrate neural tube is the generation of gradients of morphogens both from dorsally and ventrally which eventually results in dorsalization and ventralization of neural cell fates (Martí and Bovolenta, 2002; Gilbert, 2014)
This study showed that Sonic hedgehog (SHH) induces the expression of follow-up genes coding for transcription factors such as the LIM/homeodomain gene islet1 and that islet1 is not restrictively induced in motor neurons of spinal cord and hindbrain, and in non-motor neurons of the forebrain and, that SHH is active along the entire vertebrate neuraxis
Summary
A major patterning process in the developing vertebrate neural tube (i.e., central nervous system, CNS) is the generation of gradients of morphogens both from dorsally (roof plate) and ventrally (floor plate and prechordal plate) which eventually results in dorsalization and ventralization of neural cell fates (Martí and Bovolenta, 2002; Gilbert, 2014). This process leads to differentiation of various distinct neuronal types, best known in the spinal cord and hindbrain, depending on. The cross-repressive interactions of class I genes (repressed by SHH) and class II genes (activated by SHH) lead to differential gene expression for neuron identity in the ventral versus dorsal neural tube, including the ventral expression of islet
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