ABSTRACT
Atherosclerosis is a progressive, degenerative vascular disease and a leading cause of morbidity and mortality. In response to endothelial damage, platelet derived growth factor (PDGF)-BB induced phenotypic modulation of medial smooth muscle cells (VSMCs) promotes atherosclerotic lesion formation and destabilisation of the vessel wall. VSMC sensitivity to PDGF-BB is determined by endocytosis of Low density lipoprotein receptor related protein 1 (LRP1)-PDGFR ß complexes to balance receptor recycling with lysosomal degradation. Consequently, LRP1 is implicated in various arterial diseases. Having identified Tß4 as a regulator of LRP1-mediated endocytosis to protect against aortic aneurysm, we sought to determine whether Tß4 may additionally function to protect against atherosclerosis, by regulating LRP1-mediated growth factor signalling. By single cell transcriptomic analysis, Tmsb4x, encoding Tß4, strongly correlated with contractile gene expression and was significantly down-regulated in cells that adopted a modulated phenotype in atherosclerosis. We assessed susceptibility to atherosclerosis of global Tß4 knockout mice using the ApoE-/- hypercholesterolaemia model. Inflammation, elastin integrity, VSMC phenotype and signalling were analysed in the aortic root and descending aorta. Tß4KO; ApoE-/- mice develop larger atherosclerotic plaques than control mice, with medial layer degeneration characterised by accelerated VSMC phenotypic modulation. Defects in Tß4KO; ApoE-/- mice phenocopied those in VSMC-specific LRP1 nulls and, moreover, were underpinned by hyperactivated LRP1-PDGFRß signalling. We identify an atheroprotective role for endogenous Tß4 in maintaining differentiated VSMC phenotype via LRP1-mediated PDGFRß signalling.
Subject(s)
Atherosclerosis , Muscle, Smooth, Vascular , Animals , Mice , Apolipoproteins E/genetics , Becaplermin , LDL-Receptor Related Proteins , Lipoproteins, LDLABSTRACT
Vascular stability and tone are maintained by contractile smooth muscle cells (VSMCs). However, injury-induced growth factors stimulate a contractile-synthetic phenotypic modulation which increases susceptibility to abdominal aortic aneurysm (AAA). As a regulator of embryonic VSMC differentiation, we hypothesized that Thymosin ß4 (Tß4) may function to maintain healthy vasculature throughout postnatal life. This was supported by the identification of an interaction with low density lipoprotein receptor related protein 1 (LRP1), an endocytic regulator of platelet-derived growth factor BB (PDGF-BB) signaling and VSMC proliferation. LRP1 variants have been implicated by genome-wide association studies with risk of AAA and other arterial diseases. Tß4-null mice displayed aortic VSMC and elastin defects that phenocopy those of LRP1 mutants, and their compromised vascular integrity predisposed them to Angiotensin II-induced aneurysm formation. Aneurysmal vessels were characterized by enhanced VSMC phenotypic modulation and augmented PDGFR-ß signaling. In vitro, enhanced sensitivity to PDGF-BB upon loss of Tß4 was associated with dysregulated endocytosis, with increased recycling and reduced lysosomal targeting of LRP1-PDGFR-ß. Accordingly, the exacerbated aneurysmal phenotype in Tß4-null mice was rescued upon treatment with the PDGFR-ß antagonist Imatinib. Our study identifies Tß4 as a key regulator of LRP1 for maintaining vascular health, and provides insights into the mechanisms of growth factor-controlled VSMC phenotypic modulation underlying aortic disease progression.
Subject(s)
Aortic Aneurysm, Abdominal/prevention & control , Muscle, Smooth, Vascular/metabolism , Myocytes, Smooth Muscle/metabolism , Signal Transduction/drug effects , Thymosin/pharmacology , Angiotensin II/adverse effects , Angiotensin II/pharmacology , Animals , Aortic Aneurysm, Abdominal/chemically induced , Aortic Aneurysm, Abdominal/genetics , Aortic Aneurysm, Abdominal/metabolism , Becaplermin/genetics , Becaplermin/metabolism , Low Density Lipoprotein Receptor-Related Protein-1/genetics , Low Density Lipoprotein Receptor-Related Protein-1/metabolism , Male , Mice , Mice, Knockout , Receptor, Platelet-Derived Growth Factor beta/genetics , Receptor, Platelet-Derived Growth Factor beta/metabolism , Signal Transduction/genetics , Thymosin/genetics , Thymosin/metabolismABSTRACT
Restoring blood flow after myocardial infarction (MI) is essential for survival of existing and newly regenerated tissue. Endogenous vascular repair processes are deployed following injury but are poorly understood. We sought to determine whether developmental mechanisms of coronary vessel formation are intrinsically reactivated in the adult mouse after MI. Using pulse-chase genetic lineage tracing, we establish that de novo vessel formation constitutes a substantial component of the neovascular response, with apparent cellular contributions from the endocardium and coronary sinus. The adult heart reverts to its former hypertrabeculated state and repeats the process of compaction, which may facilitate endocardium-derived neovascularization. The capacity for angiogenic sprouting of the coronary sinus vein, the adult derivative of the sinus venosus, may also reflect its embryonic origin. The quiescent epicardium is reactivated and, while direct cellular contribution to new vessels is minimal, it supports the directional expansion of the neovessel network toward the infarcted myocardium. Thymosin ß4, a peptide with roles in vascular development, was required for endocardial compaction, epicardial vessel expansion, and smooth muscle cell recruitment. Insight into pathways that regulate endogenous vascular repair, drawing on comparisons with development, may reveal novel targets for therapeutically enhancing neovascularization.
