Atherothrombosis, resulting from rupture or erosion of unstable atherosclerotic plaques, is the leading cause of
death worldwide. However, the mechanisms that regulate the stability of late stage atherosclerotic lesions remain
poorly understood. Recent studies from our lab (2021 Nature Metabolism) showed that smooth muscle cell (SMC)
conditional knockout (KO) of the platelet-derived growth factor receptor-ß (PDGFRß) in ApoE-/- mice was associated
with nearly complete failure of SMC to invest into lesions or the fibrous cap. However, despite nearly complete
absence of SMC in lesions of SMC PDGFRß KO mice, surprisingly we observed no changes in lesion size or indices
of plaque stability, including the thickness of the ACTA2+ fibrous cap, following 18 weeks of Western diet (WD).
Further studies showed that: 1) contrary to dogma that Acta2+ fibrous cap cells are derived almost exclusively from
SMC, we found they account for only 60-75% of ACTA2+ cells in advanced ApoE-/- brachiocephalic (BCA) or human
coronary artery (CA) lesions with the remainder coming from endothelial cell (EC)-to-mesenchymal-to-myofibroblast
transitions (EndoMT-MFT, 15-20%) and macrophage-to-myofibroblast transitions (MMFT, 10-15%); 2) loss of SMC
investment into lesions with SMC PDGFR? KO or bone marrow transplantation (BMT) was associated with a 2-3 fold
increase in EndoMT-MFT and MMFT; 3) increased EndoMT-MFT, and MMFT did not sustain indices of stability when
WD feeding was extended to 26 weeks suggesting qualitative differences in the ability of fibrous cap cells to sustain
lesion stability depending on their origin; 4) the contribution of EC to the ACTA2+ fibrous cap increased with lesion
regression by feeding mice 18 weeks of WD followed by 12 weeks of chow diet; and 5) female but not male mice
exhibit IL1R1-dependent EndoMT. Studies herein will test the hypothesis that the contribution of EC to the ACTA2+
fibrous cap increases with lethal irradiation-BMT, SMC PDGFRß KO, aging, female sex, and/or plaque regression
resulting in: 1) increased EC proliferation and replicative senescence; 2) loss of EC integrity and/or reduced anti-
thrombotic properties; and 3) increased susceptibility to plaque erosion or rupture. Aim 1 will determine if the marked
increase in the contribution of EndoMT-MFT to the ACTA2+ fibrous cap following lethal irradiation-BMT, SMC
PDGFR? KO, or with plaque regression is associated with increased proliferation, clonal expansion, replicative
senescence, and/or dysfunction of EC. Aim 2 will determine if treatment of our SMC- or EC-lineage tracing ApoE-/-
or our novel Myh11-CreERT2/Rosa-eYFP/SR-BI¿CT/¿CT/LDLR- (¿CT) mice with advanced BCA and CA lesions with
senolytic drugs is associated with beneficial changes in indices of plaque stability, improved survival, reduced MI or
stroke, reduced dependence on EndoMT-MFT for maintenance of the ACTA2+ fibrous cap, and/or improved EC
integrity and/or function. All aims include human validation studies on stable versus high risk carotid endarterectomy
samples, and assessment of sex-dependent determinants of late-stage lesion pathogenesis. The proposed studies
will provide novel insights regarding mechanisms that regulate the cellular and extracellular matrix (ECM) composition
of the fibrous cap and may lead to development of novel therapeutic approaches for enhancing plaque stability.
