Search

Abstract Midlobular hepatocytes are proposed to be the most plastic hepatic cell, providing a reservoir for hepatocyte proliferation during homeostasis and regeneration. However, other mechanisms beyond hyperplasia have been little explored and the contribution of other hepatocyte subpopulations to regeneration has been controversial. Thus, re-examining hepatocyte dynamics during regeneration is critical for cell therapy and treatment of liver diseases. Using a mouse model of hepatocyte- and non-hepatocyte- multicolor lineage tracing, we demonstrate that midlobular hepatocytes also undergo hypertrophy in response to chemical, physical, and viral insults. Our study shows that this subpopulation also combats liver impairment after infection with coronavirus. Furthermore, we demonstrate that pericentral hepatocytes also expand in number and size during the repair process and Galectin-9-CD44 pathway may be critical for driving these processes. Notably, we also identified that transdifferentiation and cell fusion during regeneration after severe injury contribute to recover hepatic function.

Alveolarization ensures sufficient lung surface area for gas exchange, and during bulk alveolarization in mice (postnatal day [P] 4.5–14.5), alpha-smooth muscle actin (SMA)+ myofibroblasts accumulate, secrete elastin, and lay down alveolar septum. Herein, we delineate the dynamics of the lineage of early postnatal SMA+ myofibroblasts during and after bulk alveolarization and in response to lung injury. SMA+ lung myofibroblasts first appear at ∼ P2.5 and proliferate robustly. Lineage tracing shows that, at P14.5 and over the next few days, the vast majority of SMA+ myofibroblasts downregulate smooth muscle cell markers and undergo apoptosis. Of note, ∼8% of these dedifferentiated cells and another ∼1% of SMA+ myofibroblasts persist to adulthood. Single cell RNA sequencing analysis of the persistent SMA− cells and SMA+ myofibroblasts in the adult lung reveals distinct gene expression profiles. For instance, dedifferentiated SMA− cells exhibit higher levels of tissue remodeling genes. Most interestingly, these dedifferentiated early postnatal myofibroblasts re-express SMA upon exposure of the adult lung to hypoxia or the pro-fibrotic drug bleomycin. However, unlike during alveolarization, these cells that re-express SMA do not proliferate with hypoxia. In sum, dedifferentiated early postnatal myofibroblasts are a previously undescribed cell type in the adult lung and redifferentiate in response to injury.

Summary: Smooth muscle cell (SMC) accumulation is central to the pathogenesis of elastin-defective arterial diseases, including supravalvular aortic stenosis (SVAS). We previously demonstrated that elastin insufficiency activates Notch signaling in aortic SMCs. Activation of Notch is catalyzed by the enzyme gamma-secretase, but the role of catalytic subunits presenilin (PSEN)-1 or PSEN-2 in elastin aortopathy is not defined. Genetic approaches reveal that endothelial cell-specific Psen1 deletion does not improve elastin aortopathy whereas the deletion of either Psen1 in SMCs or Psen2 globally attenuates Notch pathway and SMC proliferation, mitigating aortic disease. With SMC-specific Psen1 deletion in elastin nulls, these rescue effects are more robust and in fact, survival is increased. SMC deletion of Psen1 also attenuates hypermuscularization in newborns heterozygous for the elastin null gene, which genetically mimics SVAS. Similarly, the pharmacological inhibition of PSEN-1 mitigates SMC accumulation in elastin aortopathy. These findings put forth SMC PSEN-1 as a potential therapeutic target in SVAS.

The pluripotency factor KLF4 has been described as pro-fibrotic or anti-fibrotic in various diseases. Herein, the authors show that during lung fibrosis, these distinct effects can be attributed to mesenchymal cell-type specific functions of KLF4.

Objective: The phenotypic plasticity of vascular smooth muscle cells (VSMCs) is central to vessel growth and remodeling, but also contributes to cardiovascular pathologies. New technologies including fate mapping, single cell transcriptomics, and genetic and pharmacologic inhibitors have provided fundamental new insights into the biology of VSMC. The goal of this review is to summarize the mechanisms underlying VSMC phenotypic modulation and how these might be targeted for therapeutic benefit. Methods: We summarize findings from extensive literature searches to highlight recent discoveries in the mechanisms underlying VSMC phenotypic switching with particular relevance to intimal hyperplasia. PubMed was searched for publications between January 2001 and December 2020. Search terms included VSMCs, restenosis, intimal hyperplasia, phenotypic switching or modulation, and drug-eluting stents. We sought to highlight druggable pathways as well as recent landmark studies in phenotypic modulation. Results: Lineage tracing methods have determined that a small number of mature VSMCs dedifferentiate to give rise to oligoclonal lesions in intimal hyperplasia and atherosclerosis. In atherosclerosis and aneurysm, single cell transcriptomics reveal a striking diversity of phenotypes that can arise from these VSMCs. Mechanistic studies continue to identify new pathways that influence VSMC phenotypic plasticity. We review the mechanisms by which the current drug-eluting stent agents prevent restenosis and note remaining challenges in peripheral and diabetic revascularization for which new approaches would be beneficial. We summarize findings on new epigenetic (DNA methylation/TET methylcytosine dioxygenase 2, histone deacetylation, bromodomain proteins), transcriptional (Hippo/Yes-associated protein, peroxisome proliferator-activity receptor-gamma, Notch), and β3-integrin-mediated mechanisms that influence VSMC phenotypic modulation. Pharmacologic and genetic targeting of these pathways with agents including ascorbic acid, histone deacetylase or bromodomain inhibitors, thiazolidinediones, and integrin inhibitors suggests potential therapeutic value in the setting of intimal hyperplasia. Conclusions: Understanding the molecular mechanisms that underlie the remarkable plasticity of VSMCs may lead to novel approaches to treat and prevent cardiovascular disease and restenosis.

Obstructive arterial diseases, including supravalvular aortic stenosis (SVAS), atherosclerosis, and restenosis, share 2 important features: an abnormal or disrupted elastic lamellae structure and excessive smooth muscle cells (SMCs). However, the relationship between these pathological features is poorly delineated. SVAS is caused by heterozygous loss-of-function, hypomorphic, or deletion mutations in the elastin gene (ELN), and SVAS patients and elastin-mutant mice display increased arterial wall cellularity and luminal obstructions. Pharmacological treatments for SVAS are lacking, as the underlying pathobiology is inadequately defined. Herein, using human aortic vascular cells, mouse models, and aortic samples and SMCs derived from induced pluripotent stem cells of ELN-deficient patients, we demonstrated that elastin insufficiency induced epigenetic changes, upregulating the NOTCH pathway in SMCs. Specifically, reduced elastin increased levels of γ-secretase, activated NOTCH3 intracellular domain, and downstream genes. Notch3 deletion or pharmacological inhibition of γ-secretase attenuated aortic hypermuscularization and stenosis in Eln–/– mutants. Eln–/– mice expressed higher levels of NOTCH ligand JAGGED1 (JAG1) in aortic SMCs and endothelial cells (ECs). Finally, Jag1 deletion in SMCs, but not ECs, mitigated the hypermuscular and stenotic phenotype in the aorta of Eln–/– mice. Our findings reveal that NOTCH3 pathway upregulation induced pathological aortic SMC accumulation during elastin insufficiency and provide potential therapeutic targets for SVAS.

Excess macrophages and smooth muscle cells (SMCs) characterize many cardiovascular diseases, but crosstalk between these cell types is poorly defined. Pulmonary hypertension (PH) is a lethal disease in which lung arteriole SMCs proliferate and migrate, coating the normally unmuscularized distal arteriole. We hypothesized that increased macrophage platelet-derived growth factor–B (PDGF-B) induces pathological SMC burden in PH. Our results indicate that clodronate attenuates hypoxia-induced macrophage accumulation, distal muscularization, PH, and right ventricle hypertrophy (RVH). With hypoxia exposure, macrophage Pdgfb mRNA was upregulated in mice, and LysM‑Cre mice carrying floxed alleles for hypoxia-inducible factor 1a, hypoxia-inducible factor 2a, or Pdgfb had reduced macrophage Pdgfb and were protected against distal muscularization and PH. Conversely, LysM‑Cre von-Hippel Lindaufl/fl mice had increased macrophage Hifa and Pdgfb and developed distal muscularization, PH, and RVH in normoxia. Similarly, Pdgfb was upregulated in macrophages from human idiopathic or systemic sclerosis–induced pulmonary arterial hypertension patients, and macrophage-conditioned medium from these patients increased SMC proliferation and migration via PDGF-B. Finally, in mice, orotracheal administration of nanoparticles loaded with Pdgfb siRNA specifically reduced lung macrophage Pdgfb and prevented hypoxia-induced distal muscularization, PH, and RVH. Thus, macrophage-derived PDGF-B is critical for pathological SMC expansion in PH, and nanoparticle-mediated inhibition of lung macrophage PDGF-B has profound implications as an interventional strategy for PH.

Pericytes are perivascular cells that regulate blood vessel formation and function. Here Dubrac et al. show that pericyte recruitment contributes to pathological neovascularisation in a mouse model of ischemic retinopathy, and that this depends on the regulation of PDGF-B signaling by NCK adaptor proteins.

Smooth muscle cells (SMCs) invade atherosclerotic lesions and expand, contributing to plaque progression. Here Misra et al. show that SMC-derived plaque cells come from a single SMC and integrin β3 in SMCs and macrophages regulate the fate, expansion and migration of SMCs during plaque formation.

Summary: Pulmonary hypertension is a devastating disease characterized by excessive vascular muscularization. We previously demonstrated primed platelet-derived growth factor receptor β+ (PDGFR-β+)/smooth muscle cell (SMC) marker+ progenitors at the muscular-unmuscular arteriole border in the normal lung, and in hypoxia-induced pulmonary hypertension, a single primed cell migrates distally and expands clonally, giving rise to most of the pathological smooth muscle coating of small arterioles. Little is known regarding the molecular mechanisms underlying this process. Herein, we show that primed cell expression of Kruppel-like factor 4 and hypoxia-inducible factor 1-α (HIF1-α) are required, respectively, for distal migration and smooth muscle expansion in a sequential manner. In addition, the HIF1-α/PDGF-B axis in endothelial cells non-cell autonomously regulates primed cell induction, proliferation, and differentiation. Finally, myeloid cells transdifferentiate into or fuse with distal arteriole SMCs during hypoxia, and Pdgfb deletion in myeloid cells attenuates pathological muscularization. Thus, primed cell autonomous and non-cell autonomous pathways are attractive therapeutic targets for pulmonary hypertension. : Sheikh et al. demonstrate that hypoxia-induced expression of KLF4 and HIF1-α in specialized lung arteriole SMC progenitors is required for distal migration and smooth muscle expansion, respectively. A HIF1-α/PDGF-B axis in endothelial cells non-cell autonomously regulates progenitor SMC induction, proliferation, and differentiation. The myeloid cell lineage marks SMCs. Keywords: smooth muscle biology, vascular wall, vascular biology, pulmonary artery, pulmonary hypertension, pulmonary vascular disease, vasculoproliferative disease, cardiovascular disease, endothelial-smooth muscle cell interactions, smooth muscle progenitors

Excess smooth muscle accumulation is a key component of many vascular disorders, including atherosclerosis, restenosis, and pulmonary artery hypertension, but the underlying cell biological processes are not well defined. In pulmonary artery hypertension, reduced pulmonary artery compliance is a strong independent predictor of mortality, and pathological distal arteriole muscularization contributes to this reduced compliance. We recently demonstrated that embryonic pulmonary artery wall morphogenesis consists of discrete developmentally regulated steps. In contrast, poor understanding of distal arteriole muscularization in pulmonary artery hypertension severely limits existing therapies that aim to dilate the pulmonary vasculature but have modest clinical benefit and do not prevent hypermuscularization. Here, we show that most pathological distal arteriole smooth muscle cells, but not alveolar myofibroblasts, derive from pre-existing smooth muscle. Furthermore, the program of distal arteriole muscularization encompasses smooth muscle cell dedifferentiation, distal migration, proliferation, and then redifferentiation, thereby recapitulating many facets of arterial wall development.