Biomedical subjects

G Schratt

Publications and source records attributed to G Schratt.

3 recordsLinked to original sources

PubMed2001

Serum response factor is required for immediate-early gene activation yet is dispensable for proliferation of embryonic stem cells.

Addition of serum to mitogen-starved cells activates the cellular immediate-early gene (IEG) response. Serum response factor (SRF) contributes to such mitogen-stimulated transcriptional induction of many IEGs during the G0-G1 cell cycle transition. SRF is also believed to be essential for cell cycle progression, as impairment of SRF activity by specific antisera or antisense RNA has previously been shown to block mammalian cell proliferation. In contrast, Srf(-/-) mouse embryos grow and develop up to E6.0. Using the embryonic stem (ES) cell system, we demonstrate here that wild-type ES cells do not undergo complete cell cycle arrest upon serum withdrawal but that they can mount an efficient IEG response. This IEG response, however, is severely impaired in Srf(-/-) ES cells, providing the first genetic proof that IEG activation is dependent upon SRF. Also, Srf(-/-) ES cells display altered cellular morphology, reduced cortical actin expression, and an impaired plating efficiency on gelatin. Yet, despite these defects, the proliferation rates of Srf(-/-) ES cells are not substantially altered, demonstrating that SRF function is not required for ES cell cycle progression.

Animals↗

PubMed2000

Srf(-/-) ES cells display non-cell-autonomous impairment in mesodermal differentiation.

The serum response factor (SRF) transcription factor is essential for murine embryogenesis. SRF+(-/-) embryos stop developing at the onset of gastrulation, lacking detectable mesoderm. This developmental defect may reflect cell-autonomous impairment of SRF(-/-) embryonic cells in mesoderm formation. Alternatively, it may be caused by a non-cell-autonomous defect superimposed upon inappropriate provision of mesoderm-inducing signals to primitive ectodermal cells. We demonstrate that the ability of SRF(-/-) embryonic stem (ES) cells to differentiate in vitro into mesodermal cells is indeed impaired. However, this impairment can be modulated by external, cell-independent factors. Retinoic acid, but not dimethylsulfoxide, permitted activation of the mesodermal marker gene T(Bra), which was also activated when SRF was expressed in SRF(-/-) ES cells. Embryoid bodies from SRF(-/-) ES cell aggregates also activated mesodermal marker genes, but displayed unusual morphologies and impairment in cavitation. Finally, in nude mice, Srf(-/-) ES cells readily differentiated into mesodermal cells of SRF(-/-) genotype, including cartilage, bone or muscle cells. We demonstrate that SRF contributes to mesodermal gene expression of ES cells and that SRF(-/-) ES cells display a non-cell-autonomous defect in differentiation towards mesoderm.

Animals↗

PubMed1999

MAPKAP kinase 2 phosphorylates serum response factor in vitro and in vivo.

Several growth factor- and calcium-regulated kinases such as pp90(rsk) or CaM kinase IV can phosphorylate the transcription factor serum response factor (SRF) at serine 103 (Ser-103). However, it is unknown whether stress-regulated kinases can also phosphorylate SRF. We show that treatment of cells with anisomycin, arsenite, sodium fluoride, or tetrafluoroaluminate induces phosphorylation of SRF at Ser-103 in both HeLa and NIH3T3 cells. This phosphorylation is dependent on the kinase p38/SAPK2 and correlates with the activation of MAPKAP kinase 2 (MK2). MK2 phosphorylates SRF in vitro at Ser-103 with similar efficiency as the small heat shock protein Hsp25 and significantly better than CREB. Comparison of wild type murine fibroblasts with those derived from MK2-deficient mice (Mk(-/-)) reveals MK2 as the major SRF kinase induced by arsenite. These results demonstrate that SRF is targeted by several signal transduction pathways within cells and establishes SRF as a nuclear target for MAPKAP kinase 2.

3T3 Cells↗