Discontinuous organization and specification of the lateral floor plate in zebrafish

Abstract

The floor plate is a signaling center in the ventral neural tube of vertebrates with important functions during neural patterning and axon guidance. It is composed of a centrally located medial floor plate (MFP) and a bilaterally positioned lateral floor plate (LFP). While the role of the MFP as source of signaling molecules like, e.g., Sonic Hedgehog (Shh) is well understood, the exact organization and function of the LFP are currently unclear. Based on expression analyses, the one cell wide LFP in zebrafish has been postulated to be a homogenous structure. We instead show that the zebrafish trunk LFP is discontinuously arranged. Single LFP cells alternate with p3 neuronal precursor cells, which develop V3 interneurons along the anteroposterior (AP) axis. Our mutant analyses indicate that both, formation of LFP and p3 cells require Delta-Notch signaling. Importantly, however, the two cell types are differentially regulated by Hedgehog (HH) and Nkx2.2 activities. This implicates a novel mechanism of neural tube patterning, in which distinct cell populations within one domain of the ventral neural tube are differently specified along the AP axis. We conclude that different levels of HH and Nkx2.2 activities are responsible for the alternating appearance of LFP and p3 neuronal progenitor cells in the zebrafish ventral neural tube.

nkx2.2apromotes specification and differentiation of a myelinating subset of oligodendrocyte lineage cells in zebrafish

“…We tested this using antisense MOs designed to interfere with nkx2.2a translation. Previous work showed that nkx2.2a MOs reduce or eliminate expression of genes that mark cells born within the p3 domain (Schafer et al, 2007;Kucenas et al, 2008). Accordingly, in nkx2.2a MO-injected embryos, olig2 RNA expression expanded ventrally to include cells adjacent to medial floor plate (see Supplementary Fig.…”

“…nkx2.2a MO1 (5′-CCG TCT TTG TGT TGG TCA ACG ACA T-3′) was complementary to a sequence spanning just before and including the translation start codon (Schafer et al, 2007;Kucenas et al, 2008). nkx2.2a MO2 (5′-AAG TTG CTG CAC CAG TTT GAC AAT C-3′) was complementary to a sequence in the 5′ UTR.…”

“…Near the end of embryogenesis and during early postnatal life, many OPCs stop dividing, wrap multiple axons with fine membrane extensions and express myelin genes whereas other OPCs remain in a slowly dividing, immature, nonmyelinating state (Wolswijk and Noble, 1989;Nishiyama et al, 1999). During early stages of neural development in rodents, birds and fish, p3 precursors, which lie just ventral to pMN precursors, express Nkx2.2 (Ericson et al, 1997;Briscoe et al, 1999Briscoe et al, , 2000Soula et al, 2001;Schafer et al, 2007). In chick embryos, Nkx2.2 + cells disperse from their ventral neuroepithelial origin and express markers of the oligodendrocyte lineage (Xu et al, 2000;Soula et al, 2001).…”

Subdivisions of chick diencephalic roof plate: Implication in the formation of the posterior commissure

“…Axonal guidance has been studied principally in the spinal cord, where there is a subset of cells in the ventral and dorsal midline, floor plate and roof plate respectively, that secrete several factors involved in axonal guidance (Kennedy et al, 1994;Murakami and Shirasaki, 1997;Matise et al, 1999;Kaprielian et al, 2001). The description of spatially restricted expression of various immunocytochemical and molecular markers in ventral neural tube of several vertebrate species has led to the distinction of two populations of cells in the floor plate: a centrally located medial floor plate (MFP) and a bilaterally positioned lateral floor plate (LFP; Schafer et al, 2007). Thus, in the chick ventral neural tube at Hamburger and Hamilton stage (HH) 20 (Hamburger and Hamilton, 1951), a medial region, where the cells express both SC1 and FP1 antigens, can be distinguished from lateral areas where the cells express FP1 but not SC1 Yamada et al, 1991).…”

“…The division between MFP and LFP has functional implications due to the differential expression of axonal guidance cues. In chicken, for instance, the floor plate marker genes sonic hedgehog and netrin-1 are expressed in the LFP, while Foxa2 is transiently restricted to the MFP (Marti et al, 1995;Charrier et al, 2002;Schafer et al, 2007). On the other hand, the cells of the roof plate, located at the dorsal midline, are also specialized in the secretion of molecules that control multiple aspects of the dorsal CNS development, including axonal guidance (Chizhikov and Millen, 2005).…”