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We have compared the roles of XASH-3 and NeuroD, two basic helix-loop-helix transcription factors, in the formation of primary neurons in early Xenopus embryos. When ectopically expressed in Xenopus embryos, XASH-3 and NeuroD induce ectopic primary neurons in very different spatial patterns. We show that the pattern of primary neurons induced by XASH-3 and NeuroD can be accounted for by a difference in their sensitivity to inhibitory interactions mediated by the neurogenic genes, X-Delta-1 and X-Notch-1. Both NeuroD and XASH-3 promote the expression of the inhibitory ligand, X-Delta-1. However, XASH-3 appears to be sensitive to the inhibitory effects of X-Delta-1 while NeuroD is much less so. Consequently only a subset of cells that ectopically express XASH-3 eventually form neurons, giving a scattered pattern, while the ectopic expression of NeuroD leads to a relatively dense pattern of ectopic neurons. We propose that differences in the sensitivity of XASH-3 and NeuroD to lateral inhibition play an important role during their respective roles in neuronal determination and differentiation.
Fig. 1. Pattern of NECs, and X-Delta-1
expression, in Xenopus embryos following
ectopic expression of XASH-3, NeuroD,
ICD and X-Delta-1Stu. Each panel shows a
dorsal view of a Xenopus embryo at stage
13 (neural plate stage) after staining for Xgal
(sky blue), which reveals the injected
side, and for N-tubulin RNA (dark purple)
which reveals the pattern of NECs
(A,B,D-I), or for X-Delta-1 (C). Each
embryo is oriented with anterior to the left
and posterior to the right, with the injected
side facing down. (A) Ectopic expression
of XASH-3 RNA (1.0 pg) results in a
suppression of NECs. The normal three
strips of NECs are marked on the upper
side: (m) medial, which give rise to
primary motorneurons; (i) intermediate,
which gives rise to primary interneurons;
and (l) lateral, which gives rise to primary
sensory neurons. (B) Ectopic expression
of XASH-3 (0.25 pg) results in ectopic
NECs in a salt and pepper pattern.
(C) Ectopic expression of XASH-3 (2 pg)
results in activation of X-Delta-1
expression. (D) Ectopic expression of
XASH-3 (1 pg) and X-Delta-1Stu (1 pg)
results in an ectopic and dense pattern of
NECs. (E) Lateral, ectopic expression of X-Delta-1Stu (1 pg) results in more NECs in the lateral stripe, but the size of the stripe is largely
unchanged. (F) Ectopic expression of NeuroD (0.5 pg) promotes ubiquitous and dense pattern of NECs. (G) Ectopic expression of NeuroD (0.5
pg) and an activated from of Notch ICD (0.5 pg) gives a similar pattern of NECs as with NeuroD alone. (H) Ectopic expression of ICD (0.5 pg)
blocks the formation of NECs. (I) Ectopic expression of ICD/XASH-3/X-Delta-1Stu (0.5/1.0/1.0 pg) RNA results in a suppression of NECs.
Fig. 2. Effects of XASH-3 and NeuroD on
the expression of X-Delta-1, X-Notch-1 and
N-tubulin in neuralized animal caps.
(A) Animal caps were isolated at blastula
stages from embryos injected at the twocell
stage with different RNAs, cultured to
the equivalent of stage 14 (neural plate)
and then assayed for the expression of XDelta-
1 and X-Notch-1 using RNAse
protection. EF-1a is a ubiquitously
expressed RNA and serves as a loading
control (Krieg et al., 1989). Note that, in
animal caps neuralized by Noggin (Lamb et
al., 1993), the expression of X-Delta-1 is
activated by both NeuroD and XASH-3, and this can be suppressed by the inclusion of the activated form of X-Notch-1 called ICD (Chitnis et
al., 1995). (B) Animal caps were isolated at blastula stages from embryos injected at the two-cell stage with different RNAs, cultured to the
equivalent of stage 16 (late neural plate) and then assayed for the expression of NCAM (Kintner and Melton, 1987) or for N-tubulin (Richter et
al., 1988) using RNAse protection assays. Note that NeuroD induces the expression of N-tubulin in animal caps neuralized by Noggin, as
shown by the expression of NCAM, and that this activation is not blocked by the coinjection of ICD.
Fig. 3. Model for the interactions between proneural and neurogenic
genes during primary neurogenesis in Xenopus. Diagrams in A and B
represent potential interactions between proneural and neurogenic
genes within cells of the neural plate where primary neurons are
generated. In A, proneural genes expression promotes neuronal
differentiation, but also promotes the expression of X-Delta-1. The
expression of X-Delta-1 activates Notch in neighboring cells as
shown in B, which blocks the ability of proneural genes to activate
Delta in these cells. These interactions produce a negative feedback
loop which allows the cell in A to express NeuroD, at which point it
undergoes neuronal differentiation. Conversely, the cell in B cannot
turn on NeuroD because of Notch activation, and thus fails to
differentiate.
