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X-MyT1 is a C2HC-type zinc finger protein that we find to be involved in the primary selection of neuronal precursor cells in Xenopus. Expression of this gene is positively regulated by the bHLH protein X-NGNR-1 and negatively regulated by the Notch/Delta signal transduction pathway. X-MyT1 is able to promote ectopic neuronal differentiation and to confer insensitivity to lateral inhibition, but only in cooperation with bHLH transcription factors. Inhibition of X-MyT1 function inhibits normal neurogenesis as well as ectopic neurogenesis caused by overexpression of X-NGNR-1. On the basis of these findings, we suggest that X-MyT1 is a novel, essential element in the cascade of events that allows cells to escape lateral inhibition and to enter the pathway that leads to terminal neuronal differentiation.
Figure 1.
Conserved Structural Features of MyT1-Type Zinc Finger Proteins in Vertebrate and Invertebrate Species
(A) Structural organization of MyT1-type proteins from Xenopus, human, rat, and C. elegans was predicted from the corresponding cDNAs. Closed bars indicate the position of individual C2HC zinc finger units. Dashed lines indicate cDNAs with incomplete 5â² ends. Percentage of identity in the zinc finger clusters and in the nonfinger portions is indicated.
(B) The amino acid sequence of the X-MyT1 protein was predicted from the corresponding cDNA. The two zinc finger clusters are boxed; Glu/Asp-rich region and Ser-rich regions are indicated by dashed underlining and solid underlining, respectively.
Figure 2.
X-MyT1 Expression Correlates with Primary Neurogenesis
(A) Temporal- and spatial-expression characterisitics of X-MyT1, X-NGNR-1 and N-tubulin during early Xenopusembryogenesis were analyzed by wholemount in situ hybridization with probes as indicated above each column. Stages of embryonic development are indicated on the left for each line of embryos. All embryos are shown in a dorsal view, with anterior to the right. X-NGNR-1, X-MyT1, and N-tubulin expression are activated sequentially in cells corresponding to primary neurons. This temporal order of expression correlates with a gradual sharpening of expression boundaries.
(B) The exact spatial correlation of X-MyT1, X-Delta-1, and N-tubulin expression was analyzed by sequential double-staining wholemount in situ hybridization. X-MyT1 (red) and N-tubulin (purple) are expressed in an overlapping set of scattered cells in the lateral stripe of a stage 14 embryo; some lateral cells only express X-MyT1 (top panel). X-Delta-1 (red) and X-MyT1 (purple) share identical domains of expression within the neural plate and are activated at the same time of development (bottom panel, stage 12 and 14 embryos). X-Delta-1 is also strongly expressed in an area adjacent to the blastopore.
Figure 3.
X-MyT1 Expression during Secondary Neurogenesis
(A) X-MyT1 expression in tadpole-stage embryos was analyzed by wholemount in situ hybridization. Individual panels show (a), a total view; (b), a frontal view; (c), a parasagittal section; and (d), (e), (f), and (g), transverse sections at the level of the forebrain, eye, otic vesicle, and hindbrain, respectively. X-MyT1 expression was observed in cranial ganglia, eye, olfactory placodes, pineal gland, and throughout the nervous system. In transverse sections, X-MyT1 expression can also be detected in the hypophysis, retina, otic vesicles, and neural crest cells. Within the neural tube, X-MyT1 expression is detected in a region immediately adjacent to the ventricular zone.
(B) A comparative analysis of X-NGNR-1, X-MyT1, X-Delta-1 and N-tubulin expression in the neural tube was performed by wholemount in situ hybridization. Tadpole-stage embryos were stained for expression of the genes indicated in transverse sections prepared at the level of the hindbrain. The different domains of expression correlate with the progress of neuronal differentiation.
Figure 4.
X-MyT1 Expression Is Positively Regulated by X-NGNR-1 and Negatively Regulated by Lateral Inhibition
Xenopus embryos were injected into one cell at the two-cell stage with synthetic mRNA encoding X-NGNR-1, Notch ICD, X-Delta-1Stu, or a combination of X-NGNR-1 and Notch ICD (as indicated). Each panel shows a dorsal view of stage 14 embryos stained with X-gal (light blue, revealing the injected side) and for X-MyT1 gene expression (purple). Microinjection of X-NGNR-1 results in strong ectopic expression of X-MyT1. Microinjection of the activated form of X-Notch-1 (Notch ICD) blocks X-MyT1 expression, while ectopic expression of X-Delta-1Stu results in an increase in the density of X-MyT1-expressing cells. Coexpression of Notch ICD blocks X-NGNR-1âmediated activation of X-MyT1.
Figure 5.
X-MyT1 and bHLH Proteins Cooperate to Stimulate N-tubulin Gene Transcription
(A) X-MyT1 and XASH-3 cooperate to stimulate N-tubulin gene transcription in embryos and in animal cap explants. Embryos were injected with mRNAs encoding LacZ (control), X-MyT1, XASH-3, or a combination of XASH-3and X-MyT1 (as indicated). Injected
embryos at stage 14 (upper series) or animal caps isolated from injected blastula-stage embryos and cultured to the equivalent of tailbud stages (lower series) were stained for N-tubulin expression.
(B) Combinations of either X-MyT1 and XASH3 or X-MyT1 and X-NGNR-1 can bypass lateral inhibition in respect to N-tubulin gene expression. Embryos were injected with single or combined RNA preparations (as indicated) and stained for N-tubulin expression. ICD is the activated form of X-Notch-1.
Figure 6.
The Two Zinc Finger Clusters in X-MyT1 Exhibit Equal DNA-Binding Specificity
(A) A consensus DNA-recognition sequence was obtained via affinity selection using recombinant X-MyT1 fusion proteins containing either the first, the second, or both C2HC finger clusters. Numbers indicate the percentage at which a given nucleotide is found in the corresponding position of the affinity-selected oligonucleotides. Selected genomic fragments contain multiple copies of identical simple repeat elements.
(B) DNAse I footprinting analysis of a selected genomic fragment (left panel) and of a selected 26-mer (right panel) identifies the core-consensus recognition sequence. X-MyT1 fusion proteins utilized are indicated at the top of each lane.
Figure 7.
X-MyT1 Functions as a Transcriptional Activator and Is Required for N-tubulin Expression in Xenopus Embryos
(A) A CAT reporter construct containing two copies of the core-consensus X-MyT1-recognition element in front of the minimal TK promoter was cotransfected into Hela cells together with increasing amounts of an expression plasmid encoding either full-length X-MyT1 or with the vector alone (CMV). Levels of CAT reporter activation obtained are indicated along with standard deviations.
(B) Epitope/NLSâtagged X-MyT1 deletion mutants were tested either in transient transfection assays or in Xenopus embryos by microinjection of the corresponding RNAs in combination with either XASH-3 or X-NGNR-1 plus Notch ICDâencoding mRNAs (as in Figure 5). The transactivation capacity of the different X-MyT1 variants is shown as percentage of wild type (CAT activation). The ability to induce ectopic N-tubulin expression in Xenopus embryos is indicated by (+).
(C) Dominant-negative constructs derived from X-MyT1 interfere with N-tubulin gene transcription in Xenopus embryos. Manipulated embryos (stage 14) are shown from a dorsal view after staining with X-gal (light blue, indicating the injected side) and for N-tubulin expression (purple). Microinjection of mRNAs encoding either the
epitope/NLSâtagged central domain of X-MyT1 ([480â787], as illustrated in [B]) or the first one of the two zinc finger clusters in X-MyT1 fused to the engrailed repressor domain ([383â484-engR], as illustrated in [B]) leads to a decrease in the number of N-tubulin-expressing cells in the injected side of the embryos. Microinjection of X-NGNR-1 mRNA along with an excess of X-MyT1-engR mRNA results in a severe reduction of N-tubulin expression, as compared to the level of N-tubulin expression induced by X-NGNR-1alone.
