Many studies have characterized the transcriptional responses to signaling events since array technology was developed. For example, Fambrough et al. (1999) addressed the question of signaling specificity by comparing the transcriptional responses downstream of RTKs in cultured cells. Kit, Pdgfrp and Fgfr1 were found to induce the transcription of the same set of genes in this system (with some quantitative differences), whereas Egfr induced transcriptional responses that differed both qualitatively and quantitatively from these other RTKs. The disruption of effector binding sites on Pdgfrp did not significantly affect its transcriptional response in these experiments, consistent with what was subsequently observed in vivo (Tallquist et al., 2003).
Biologically relevant transcriptional profiling relies on the selection of informative tissue or cell samples. In a recent screen for Wnt target genes, comparative expression analysis was performed using gastrulation-stage wild-type and fi-catenin mutant mouse embryos. In addition to known Wnt target genes, several novel targets were identified in this study, including components of other signaling pathways (e.g., Notch) and genes expressed in domains of Wnt reporter activity during gastrulation. Some target genes (Grsfl, Fragilis) were further validated as Wnt-associated genes in vivo: embryos derived from RNAi knockdown embryonic stem cells recapitulated aspects of Wnt mutant phenotypes (Lickert et al., 2005).
In whole-embryo analyses, it is difficult to discern direct targets of signaling pathways from transcriptional changes that are secondary to developmental aberrations. Furthermore, different cell types and developmental contexts may respond to signals with distinct responses. For these reasons, profiling experiments would ideally use homogeneous cell populations that have not been immortalized or otherwise modified from their native state. High-fidelity cDNA amplification techniques are being developed to enable the profiling of single cells and small cell populations. This and similar technical advances will enable researchers to identify the transcriptional targets of signaling events in spatially or temporally restricted niches within developing embryos.
The results of profiling studies need to be substantiated in functional assays that demonstrate the significance of identified targets in mediating relevant cellular responses. To facilitate the transition from expression analysis to functional validation, Chen et al. (2004b) generated a microarray of cDNAs representing genes that were randomly mutated by retroviral gene trapping in ES cells. This chip—or gene trap array—can be used to profile transcriptional changes in wild-type versus mutant cells/tissues, uninduced versus induced cells, or cells at different stages of differentiation. Mutant mice can then be generated from archived mutant ES cells for the analysis of putative target genes in vivo. In an initial study, the gene trap array was used to assess transcriptional responses of mouse embryonic cells to Pdgfra versus Pdgfrp stimulation (Chen et al., 2004b). The functions of several novel Pdgf target genes identified, and their genetic interactions with Pdgfrs were then addressed in vivo. Results of these studies implicated Pdgfs in the modulation of signaling by other secreted molecules (e.g., sphingosine) identified the transcriptional targets required for specific aspects of Pdgf-dependent development, and suggested novel postnatal roles of Pdgf signaling (Schmahl et al., 2007).
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