Degree

Doctor of Philosophy (PhD)

Department

School of Animal Sciences

Document Type

Dissertation

Abstract

In response to myocardial infarction (MI), cardiac fibroblasts (CFs) activate, proliferate, and differentiate into myofibroblasts that produce extracellular matrix (ECM) to form a stabilizing scar. While essential for repair, prolonged CFs activation can drive maladaptive remodeling. Thus, understanding CF heterogeneity and its regulation is key to limiting fibrosis without disrupting healing.

Smooth muscle alpha-actin (SMαA), encoded by Acta2, is a common marker of myofibroblast differentiation, but its functional role in cardiac fibroblasts and post-MI repair was unclear. In Chapter 2, we addressed this by creating a tamoxifen-inducible, cardiac fibroblast-specific Acta2 knockout (KO) mouse model. Surprisingly, Acta2 deletion did not affect post-MI survival, cardiac function, or infarct histology. Acta2-null myofibroblasts retained normal proliferation, migration, contractility, and filamentous actin levels. This was linked to compensatory upregulation of other actin isoforms, especially Actg2 and Acta1, with cytoplasmic isoforms predominating. In addition, we found that myocardin-related transcription factor-A is critical for myofibroblast differentiation but is not required for the compensatory effects of non-Acta2 isoforms. These findings indicate that Acta2 is not essential for cardiac fibroblast function or post-MI repair due to functional redundancy among actin isoforms.

In chapter 3, snMultiomic profiling across post-MI stages identified Runx1 as a key regulator of cardiac fibroblast (CF) dynamics. Using a fibroblast-specific Runx1 KO model (Pdgfrα-CreERT2), we observed changes in CF and immune cell gene expression over time. By day 3 post-MI, wild-type hearts showed enrichment of cell cycle and catabolic pathways compared to Runx1 KO. By day 7, Runx1 KO reduced Pleiotrophin (Ptn) expression, affecting CF–immune interactions, and downregulated chondrogenic genes including Comp, Acan, Col2a1, and Lox. GRN and epigenetic analysis confirmed Comp, Ptn, Lox, Cemip, and postn are direct Runx1 targets via promoter and enhancer regulation. Interestingly, Comp expression remained suppressed in populations lacking Runx2 compensation at 4 weeks. Functionally, Runx1 deletion in CFs reduced cardiac rupture in males, improved cardiac function and hypertrophy, and lowered SMαA expression at day 7. Runx1 deficiency reduced CF proliferation and chondrogenic capacity, while maintaining their motility, contractile function, and ECM-stabilizing properties.

Date

6-25-2025

Committee Chair

Fu Xing

DOI

10.31390/gradschool_dissertations.6824

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Available for download on Wednesday, June 23, 2032

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