The basic helix–loop–helix olig3 establishes the neural plate boundary of the trunk and is necessary for development of the dorsal spinal cord

Abstract

olig genes encode a previously unrecognized group of vertebratespecific basic helix-loop-helix transcription factors. As shown in mice, chickens, and zebrafish, two members of this group, olig1 and olig2, are involved in the differentiation of motoneurons and oligodendrocytes, but nothing is known about the role of the third member, olig3. Here, we show that olig3 plays an essential role in the establishment of the neural crest-lateral neural plate boundary. In zebrafish embryos, morpholino-induced olig3 inactivation dramatically increases the number of neural crest cells, but lateral neural plate fates (interneurons and astrocytes) are missing. Zebrafish swirl mutants that have impaired bone morphogenetic protein signaling and lack neural crest cells display an expanded olig3 expression domain. Moreover, olig3 is up-regulated in mindbomb mutants lacking the neural crest because of an impaired notch signaling, and olig3 repression in such mutants rescues the neural crest. In addition, olig3 regulates ngn1 and deltaA expression in interneuron precursors. Our results indicate that olig3 has an essential proneural activity in the dorsal spinal cord and cooperates with the Delta͞Notch regulatory loop to establish the boundary between the neural crest and the lateral neural plate. Thus, a proper regulation of the olig gene family is essential for the formation of three cell types (oligodendrocytes, astrocytes, and neural crest) that are unique to vertebrates.T he spinal cord of vertebrates is originated by remodeling of the neural plate. Its general plan comprises motoneurons located ventrally and interneurons in a more dorsal position. In zebrafish, Rohon-Beard (RB) cells (primary sensory neurons) are generated at the outer border of the neural plate (within the neural crest domain) and migrate in the dorsal spinal cord, whereas cells of the dorsal root ganglia (also originated in the neural crest) migrate ventrally without entering the spinal cord. Two main signaling pathways are thought to establish the dorsoventral patterning of the embryonic neural tissue: bone morphogenetic protein (BMP) and Hedgehog (Hh). Hh signaling regionalizes the ventral neural tube and restricts the expression of some genes to dorsal regions (1). In parallel, BMP signaling determines a gradient of positional information throughout the entire neural plate that defines the establishment of dorsal and intermediate neuronal cell types of the spinal cord (2). The current view is that BMP and Hh morphogenetic activities generate zones of competence within which other factors will subsequently establish different cell fates. In this regard, components of the Delta͞Notch signaling pathway are involved in the generation of different neuronal and glial subtypes with a mechanism called lateral specification. Components of the Delta͞Notch genetic loops are often members of the basic helix-loop-helix (bHLH) family of transcription factors. The Neurogenin (Ngn) subfamily of bHLH is critical to establish the neurogenic program and maintain the Delta͞No...

Midkine‐b regulates cell specification at the neural plate border in zebrafish

“…Furthermore, it was shown that the Delta-Notch downstream component Olig3 is implicated in cell fate decisions between ncc and sensory neuron fates at the neural plate border (Filippi et al, 2005). As mdkb affects formation of both cell types after overexpression and gene knockdown, we tested whether lateral inhibition might influence mdkb expression by looking at Delta-Notch deficient mib mutants.…”

“…Delta-Notch signaling by means of the basic helix-loop-helix transcription factor Olig3 is crucial for establishing cell fate decision into the ncc vs. sensory neuron lineage at the neural plate border (Filippi et al, 2005). To investigate whether mdkb expression is influenced by Delta-Notch signaling, we analyzed its expression in zebrafish mindbomb (mib ta52b ) mutants.…”

“…The inhibition of Wnt activity, for example, after expression of a dominant-negative Tcf3 variant, abolishes ncc formation, but has no influence on the number and appearance of sensory neurons (Lewis et al, 2004). On the other hand, loss of Delta-Notch activity in mib mutants results in an excess of sensory neurons at the expense of ncc (Cornell and Eisen, 2000;Itoh et al, 2003), and this is probably mediated by Olig3 (Filippi et al, 2005). This suggests that these pathways regulate cell fate decisions in an already existing pool of ncc/sensory neuron precursor cells.…”

“…Studies in zebrafish showed that also the Delta-Notch pathway is required for the correct establishment of ncc and the cell fate decision between ncc and Rohon-Beard (RB) sensory neuron formation at the neural plate border (Cornell and Eisen, 2000). In addition, several transcription factors have recently been implicated in the specification of different cell fates at the border of the zebrafish neural plate, including prdm1/ blimp-1 (Roy and Ng, 2004;Hernandez-Lagunas et al, 2005) and olig3 (Filippi et al, 2005).…”

The bHLH transcription factor Olig3 marks the dorsal neuroepithelium of the hindbrain and is essential for the development of brainstem nuclei

“…The third member, Olig3, is expressed in defined cell populations in the developing neural tube, among them dorsal progenitors (Takebayashi et al, 2002b). Recent work has demonstrated that Olig3 expression in the spinal cord is controlled by dorsal patterning signals, and that Olig3 is required to determine neuronal fates in the spinal cord of mice and zebrafish (Filippi et al, 2005;Muller et al, 2005;Zechner et al, 2007).…”