A study on the TFIID subunit TAF4

Abstract

Transcription is a fundamental process in the regulation of gene expression. It has long been generally believed that certain so called basal factors of the transcriptional machinery are ubiquitously expressed. During recent years however, this view has changed and several examples of cell- and promoter-specific roles for basal transcription factors have been demonstrated.

In this thesis, we provide evidence for cell- and context-specific roles for a component of the basal transcription factor complex TFIID - TAF4 - in transcription and neuronal differentiation.

In the first study, we identified the mouse TAF4 gene and investigated its expression during embryonic development and in the adult. The results demonstrated that TAF4 gene expression was enriched in the central nervous system and the expression pattern showed clear spatial and temporal variations. In addition, reporter assays using transient transfections of TAF4 in cell lines indicated that TAF4 could act as a positive regulator of the activity of several neuronal promoters.

In the second paper, the presence and function of several alternative isoforms of TAF4 was characterized. The expression levels of the alternative isoforms varied in a cell- and tissue-specific manner. Overexpression in retinoic acid treated cell lines showed that these truncated forms of TAF4 generally had a dominant negative effect on retinoic acid receptor-mediated transcription. The results suggest a novel, additional level of complexity in TAF4 and nuclear receptor-mediated transcription.

In the third study, we investigated a putative role for TAF4 in differentiation of cortical embryonic neural stem cells (eNSC). We found that the expression of TAF4 protein was high in fibroblast growth factor-2 (FGF2)-expanded primary cortical eNSC. Whereas astrocytic differentiation of eNSC had little effect on TAF4 protein levels or subcellular distribution, platelet-derived growth factor (PDGF) mediated neuronal differentiation was associated with a marked decrease in TAF4 protein levels. However, overexpression of TAF4 did not show any significant effect on the PDGF-mediated differentiation of eNSC. We speculated that the lack of effect could be due to insufficient levels of required co-factors.

A protein-protein interaction screen using a domain previously showed to be important for nuclear receptor transcriptional regulation (Paper II) identified the signaling protein RanBPM as a putative co-factor of TAF4. We found that high levels of TAF4 and RanBPM protein co-localized in the nuclei of neural progenitors in vivo and in vitro. Interestingly, cotransfections of TAF4 and RanBPM led to a significant increase in the number of primary neurite processes but no increase in total neurite length, whereas RanBPM and a TAF4 construct lacking the RanBPM interacting domain had no significant effect. These results demonstrate how two factors considered to be relatively general in function can influence temporally specific events such as neuritogenesis during neuronal differentiation.

To further increase the understanding of the mechanisms underlying TAF4 function, we performed a gene profiling study using TAF4-deficient cells to identify putative downstream targets of TAF4. This study revealed changes in expression levels of a number of genes associated with early and late neuronal differentiation that may underlie cell- and context-specific effects of TAF4 during brain development.