A redox-active splice variant of NADPH oxidase 4 as a potential regulator of cardiomyocyte proliferation in the adult heart

Student thesis: Doctoral ThesisDoctor of Philosophy


Background – The NADPH oxidases (Noxs) are a family of reactive oxygen species (ROS)-generating enzymes with crucial roles in the regulation of redox signalling pathways, which have been implicated in several physiological processes including cellular differentiation, proliferation, and survival. Since the heart has the highest oxygen consumption among body organs, redox-mediated signalling is central to both cardiac function as well as pathophysiology. Nox4 is expressed in cardiomyocytes and has been shown to mediate distinct adaptive phenotypes in the heart in response to chronic disease stress. We previously identified that a splice variant of Nox4, Nox4D, is nuclear-localised, capable of ROS production, and is associated with increased proliferation of neonatal cardiac myocytes. The aim of this project was to investigate the effects of Nox4D in adult cardiomyocytes in vivo. 
Methods and Results – We generated mice with inducible cardiomyocyte-specific overexpression of Nox4D (Ind-csNox4D). After tamoxifen induction in adult mice, Ind-csNox4D showed a 10-fold increase in Nox4D protein levels in the heart compared to controls, whilst echocardiography revealed similar cardiac structure and function. No changes were found in baseline cardiomyocyte cell cycling or redox signalling as assessed by EdU nucleotide analog incorporation and the expression of cell cycle genes and pathways. The impact of Ind-csNox4D on the response to myocardial infarction (MI) induced by permanent left anterior descending coronary artery (LAD) ligation was next determined. At 4 weeks post-MI, Ind-csNox4D mice demonstrated region-specific improvements in contractile function, but no differences were found in global left ventricular (LV) function and volumes compared to control mice. This was consistent with a partial protective response in Ind-csNox4D hearts during postinfarction remodelling, but there were no differences in the gene expression of markers of the cell cycle versus control hearts in either the infarct border zone or remote zone. However, using a model of cultured adult cardiomyocytes stimulated to induce dedifferentiation, we found that Nox4D overexpression is associated with increased cell cycle progression compared to control cells. 
Conclusions – In vitro overexpression of Nox4D increases proliferation of neonatal cardiomyocytes but this is unchanged in adult hearts. Induction of cardiomyocyte-specific overexpression of Nox4D in adult mice is well tolerated, capable of mediating a regionalised reparative and functional response in the setting of post-MI remodelling but insufficient to fully rescue recovery. Further research is needed to understand whether the proliferative effects of Nox4D in vitro may be realised in the adult heart.
Date of Award1 Oct 2019
Original languageEnglish
Awarding Institution
  • King's College London
SupervisorAjay Shah (Supervisor) & Els Henckaerts (Supervisor)

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