Project 7: Hypoxic regulation of ArhGAP29 gene expression and RhoGTPases in cardiac fibroblasts

PI Göttingen: D.M. Katschinski, S. Lutz; PI London: A. Ridley; PhD: L. Leinhos - finished Dr. rer. nat.

Scientific background and preliminary results

Following myocardial infarction (MI) fibroblasts accumulate in the hypoxic tissue and add to scar formation and cardiac fibrosis that causes distorted organ architecture and function but also microvessel growth in the hibernating myocardium. The underlying molecular mechanisms for these changes after MI have not been fully clarified; however, RhoGTPases are regarded as major regulators. RhoA signaling in cardiomyocytes protects against stress-induced heart failure but facilitates cardiac fibrosis; the abnormal activation of the Rho-signaling pathway in cardiac fibroblasts has been recognized in cardiovascular pathophysiology most importantly in the response of the heart to ischemia/hypoxia. RhoGTPases like RhoA are main regulators of the actin cytoskeleton; however, they also effect gene expression. This is mediated by the transcription factors MRTF (myocardin-related transcription factor) -A and SRF (serum response factor), which are thought to be key inducers of gene programs that influence e.g. fibrosis and scar formation after myocardial infarction. MRTF-A and SRF induced target genes like CCN1 and CCN2 (which is alternatively named connective tissue growth factor, CTGF). CCN1 and CCN2 are significantly shaping cardiac fibrosis but also microvessel growth (via CCN1) and maturation (via CCN2/CTGF)]. Most interestingly family members of the CCN family have been associated with the cellular response towards hypoxia/ischemia. For example, CTGF is recognized as an important hypoxia-inducible protein after MI. Up to now, the molecular mechanisms underlying the controlled activation of Rho-signaling in ischemia/hypoxia including the regulation of MRTF-A/SRF and their targets genes and the connection to the transcriptional master regulator of the cellular hypoxic response, i.e. the Hypoxia inducible factor (HIF) transcription factor are not well explored. We have recently identified the hypoxia-induced expression of the RhoA regulator ArhGAP29. The expression of ArhGAP29 is induced in hypoxia in a plethora of hypoxic cells (cell lines as well as primary cells) and in hearts after MI. This might offer the so far missing link between the activation of the transcriptional master regulator HIF and regulated RhoA activity in hypoxia/ischemia.

Hypotheses of the PhD project

Based on our preliminary data we hypothesize that ArhGAP29 is critically involved in fine tuning RhoA activity and the MRFT/SRF gene programm in hypoxia/ischemia. Thus, ArhGAP29 might hold potential for modulating cardiac remodeling under pathological conditions.

In the project the PhD student will test the hypotheses that

1. RhoA activity needs to be titrated in hypoxia to prevent overactive cytoskeletal changes and RhoA dependent gene expression mediated by MRTF-A and SRF.

2. The hypoxic induction of ArhGAP29 plays a role in fibroblasts for cardiac function and in the fibrotic response in hypoxia/ischemia.

3. ArhGAP29 expression in hypoxia is induced in a HIF-1 dependent manner.

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