Chronic mTOR activation induces a degradative smooth muscle cell phenotype
Guangxin Li, … , Jay D. Humphrey, George Tellides
Guangxin Li, … , Jay D. Humphrey, George Tellides
Published February 10, 2020
Citation Information: J Clin Invest. 2020;130(3):1233-1251. https://doi.org/10.1172/JCI131048.
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Research Article Vascular biology

Chronic mTOR activation induces a degradative smooth muscle cell phenotype

Abstract

Smooth muscle cell (SMC) proliferation has been thought to limit the progression of thoracic aortic aneurysm and dissection (TAAD) because loss of medial cells associates with advanced disease. We investigated effects of SMC proliferation in the aortic media by conditional disruption of Tsc1, which hyperactivates mTOR complex 1. Consequent SMC hyperplasia led to progressive medial degeneration and TAAD. In addition to diminished contractile and synthetic functions, fate-mapped SMCs displayed increased proteolysis, endocytosis, phagocytosis, and lysosomal clearance of extracellular matrix and apoptotic cells. SMCs acquired a limited repertoire of macrophage markers and functions via biogenesis of degradative organelles through an mTOR/β-catenin/MITF–dependent pathway, but were distinguishable from conventional macrophages by an absence of hematopoietic lineage markers and certain immune effectors even in the context of hyperlipidemia. Similar mTOR activation and induction of a degradative SMC phenotype in a model of mild TAAD due to Fbn1 mutation greatly worsened disease with near-uniform lethality. The finding of increased lysosomal markers in medial SMCs from clinical TAAD specimens with hyperplasia and matrix degradation further supports the concept that proliferation of degradative SMCs within the media causes aortic disease, thus identifying mTOR-dependent phenotypic modulation as a therapeutic target for combating TAAD.

Authors

Guangxin Li, Mo Wang, Alexander W. Caulk, Nicholas A. Cilfone, Sharvari Gujja, Lingfeng Qin, Pei-Yu Chen, Zehua Chen, Sameh Yousef, Yang Jiao, Changshun He, Bo Jiang, Arina Korneva, Matthew R. Bersi, Guilin Wang, Xinran Liu, Sameet Mehta, Arnar Geirsson, Jeffrey R. Gulcher, Thomas W. Chittenden, Michael Simons, Jay D. Humphrey, George Tellides

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Figure 4

Tsc1 deletion in SMCs causes impaired elastogenesis and greater elastolysis.

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Tsc1 deletion in SMCs causes impaired elastogenesis and greater elastol...
Tsc1fl/fl Myh11-CreERT2 mT/mG mice were treated with tamoxifen (Tmx) or vehicle (Veh), while Myh11-CreERT2 mT/mG (Tsc1+/+) and Tsc1fl/fl Myh11-CreERT2 mT/mG (Tsc1−/−) mice were treated with tamoxifen at 1.5 weeks of age. (A) Transcript expression in thoracic aortas of 3-week-old mice relative to Hprt (n = 6–7). (B) Elastin (white) and nuclei (blue) in aortic SMC cultures from 3-week-old mice (scale bar: 100 μm) and (C) quantified as mean fluorescence intensity (MFI) (n = 12). (D) Elastic fibers labeled with Alexa Fluor 633 hydrazide (AF633) in thoracic aortas of 12-week-old mice untreated or treated with elastase ex vivo (scale bar: 100 μm) and (E) the relative intensity was quantified (n = 7–8). (F) Relative intensity of cleavage products from BODIPY FL–conjugated DQ elastin incubated with thoracic aorta lysates from 24-week-old mice for 2–12 hours (n = 6). (G) Flow cytometry for MMPSense 645 activation by GFP+ SMCs isolated from ascending (Asc) or descending (Desc) aortas of 12- and 24-week-old mice and (H) quantified at 24 weeks (n = 4). (I) Immunostaining for MMP2 (white) with RFP, GFP, and DAPI overlay in ascending aortas at 24 weeks (scale bar: 100 μm) and (J) quantified in ascending and descending aortas of 12- and 24-week-old mice (n = 8). (K) Flow cytometry for MMP2 expression in GFP+ SMCs and RFP+ cells isolated from thoracic aortas of 24-week-old mice and (L) quantified at 24 weeks (n = 3–5). Data are represented as individual values with mean ± SEM bars or as box-and-whisker plots with interquartile range, median, minimum, and maximum. Additional experimental details can be found in the supplemental methods. *P < 0.05; **P < 0.01; ***P < 0.001 by 2‑way ANOVA (A, E, F, and J) or t test (C, H, and L).