Effects Of The Acta2 R258C Mutation On Vascular Smooth Muscle Cell Phenotype And Properties

Author

Katerina L. Byanova, The University of Texas Graduate School of Biomedical Sciences at HoustonFollow

Date of Graduation

8-2012

Document Type

Thesis (MS)

Program Affiliation

Cell and Regulatory Biology

Degree Name

Masters of Science (MS)

Advisor/Committee Chair

Dianna M. Milewicz, MD, PhD

Committee Member

Fernando Cabral, PhD

Committee Member

Guangwei Du, PhD

Committee Member

Jeffrey Frost, PhD

Committee Member

Ali J Marian, MD

Abstract

Thoracic Aortic Aneurysms and Dissections (TAAD) are the fifteenth leading cause of death in the United States. About 15% of TAAD patients have family history of the disease. The most commonly mutated gene in these families is ACTA2, encoding smooth muscle-specific α-actin. ACTA2 missense mutations predispose individuals both to TAAD and to vascular occlusive disease of small, muscular arteries.

Mice carrying an Acta2 R258C mutant transgene with a wildtype Acta2 promoter were generated and bred with Acta2^-/- mice to decrease the wildtype: mutant Acta2 ratio. Acta2^{+/+ R258C TG}mice have decreased aortic contractility without aortic disease. Acta2^{+/- R258C TG} mice, however, have significant aortic dilatations by 12 weeks of age and a hyperproliferative response to injury. We characterized smooth muscle cells (SMCs) from bothmouse models under the hypothesis that mutant α-actin has a dominant negative effect, leading to impaired contractile filament formation/stability, improper focal adhesion maturation and increased proliferation.

Explanted aortic SMCs from Acta2^{+/+ R258C TG} mice are differentiated - they form intact filaments, express higher levels of contractile markers compared to wildtype SMCs and have predominantly nuclear Myocardin-Related Transcription Factor A (MRTF-A) localization. However, ultracentrifugation assays showed large unpolymerized actin fractions, suggesting that the filaments are brittle. In contrast, Acta2^{+/- R258C TG} SMCs are less well-differentiated, with pools of unpolymerized actin, more cytoplasmic MRTF-A and decreased contractile protein expression compared to wildtype cells. Ultracentrifugation assays after treating Acta2^{+/- R258C TG}SMCs with phalloidin showed actin filament fractions, indicating that mutant α-actin can polymerize into filaments.

Both Acta2^{+/+ R258C TG}and Acta2^{+/- R258C TG}SMCs have larger and more peripheral focal adhesions compared to wildtype SMCs. Rac1 was more activated in Acta2^{+/+ R258C TG}SMCs; both Rac1 and RhoA were less activated in Acta2^{+/- R258C TG} SMCs, and FAK was more activated in both transgenic SMC lines compared to wildtype. Proliferation in both cell lines was significantly increased compared to wildtype cells and could be partially attenuated by inhibition of FAK or PDGFRβ. These data support a dominant negative effect of the Acta2 R258C mutation on the SMC phenotype, with increasing phenotypic severity when wildtype: mutant α-actin levels are decreased.

Keywords

TAAD, smooth muscle, ACTA2