Yan Li

Pulmonary arterial hypertension (PAH) is characterized by excessive remodeling of the proximal and distal arterioles, driven by endothelial cell apoptosis and uncontrolled mural cell proliferation. Increasing evidence suggests an important role of inflammation in PAH, but 1 crucial part of the immune system, the pulmonary lymphatics, has been largely overlooked. Patients with idiopathic PAH often develop abnormal tertiary lymphoid structures adjacent to remodeled arteries, yet the role of lymphatic vessels in PAH pathogenesis and vascular remodeling remains unclear. Using a mouse model to specifically label lymphatic endothelial cells (Prox1-CreERT2::Rosa26-LSL-tdT), we found that pulmonary lymphatic vessels proliferated and dilated in response to hypoxia. We further showed that Vegfr3 inhibition using MAZ51 reduced hypoxia induced lymphangiogenesis and was associated with severe pulmonary hypertension. Additionally, lymphatic endothelial cell-specific deletion of Vegfr3 (Prox1-CreERT2::Vegfr3 ^fl/fl ) prevents adaptive lymphangiogenesis and exacerbates pulmonary hypertension, resulting in right ventricular hypertrophy. Comparative single-cell RNA sequencing analysis of human idiopathic PAH and hypoxia-induced pulmonary hypertension mouse lungs revealed upregulation of CD74/Cd74 in lymphatic endothelial cell clusters, and in humans this was associated with enhanced expression of FLT4/VEGFR3 and downstream mediators in the MEK/ERK signaling pathway. Consistently, increased CD74 expression was observed in lymphatic vessels in human idiopathic PAH lung sections, and CD74 overexpression in human lymphatic cells was associated with impaired barrier permeability and coexpression of VEGFR3. Together, these findings provide novel insights into the role of lymphatic Cd74/CD74 activation in pulmonary hypertension development and suggest that therapies aimed at augmenting lymphatic function may improve outcomes in patients with PAH.

BACKGROUND: ADAR1 (adenosine deaminase acting on RNA 1) catalyzes the conversion of adenosine to inosine in double stranded RNAs (dsRNAs), which is critical to prevent autoinflammatory responses mediated by activation of the type I IFN (interferon) signaling. We define the role of ADAR1-dependent RNA editing in IFNβ activation in pulmonary arterial smooth muscle cells (PASMCs) from idiopathic pulmonary arterial hypertension (IPAH), a devastating disease leading to right heart failure and death. METHODS: RNA editing levels were analyzed in PASMCs from IPAH patients versus healthy controls. A conditional transgenic mouse model, Adar1 ^{SMC - KO} , was generated by knocking out Adar1 selectively in Sma (smooth muscle actin)–expressing cells, followed by 3 weeks of hypoxic exposure to induce pulmonary hypertension (PH). RESULTS: PASMCs from patients with IPAH displayed decreased levels of ADAR1 mRNA and isoform p150 protein, accompanied by accumulated dsRNA compared with healthy PASMCs. ADAR1 knockdown in PASMCs upregulated MDA5 (melanoma differentiation-associated protein 5), PKR (protein kinase R), IFNβ, and IFN-stimulated genes. Compared with controls in vivo, hypoxic Adar1 ^{SMC -KO} mice developed severe PH, as evidenced by excessive vascular remodeling in distal arterioles and increased endothelium leakage, resulting in elevated right ventricular systolic pressure and right ventricular hypertrophy. Mechanistically, Ifnβ signaling in Adar1 ^SMC-KO induced the recruitment of macrophages, enhancing pulmonary artery muscularization. Pharmacological treatment with Pkr-relevant inhibitor 2BAct decreased Ifnβ and macrophages, thus attenuating pulmonary hypertension (PH) in hypoxic Adar1 ^SMC-KO mice. CONCLUSIONS: Our study describes a fundamental molecular mechanism underlying the progression of PH. We highlight the detrimental role of innate immune responses, where smooth muscle cell and context-specific RNA editing, along with the sensing of dsRNA, mediate disease progression and excessive vascular remodeling. This finding suggests that targeting PKR could be the new therapeutic strategy for treating pulmonary arterial hypertension.

Pericytes are mural cells that line capillaries throughout the brain, retina, lung, and other organs, where they support capillary homeostasis through direct contact and paracrine crosstalk with capillary endothelium. Despite being described more than a century ago, their contributions to health and vascular diseases remain unclear, largely due to the difficulty of definitive identification. Their inherent plasticity, as well as shared markers and close lineage relationships with other mural cells, results in overlap in identification and underrepresentation in single-cell data sets. Emerging evidence reveals that pericytes play a critical role in the vascular remodeling characteristics of pulmonary hypertension, via mechanisms involving smooth muscle–like phenotypic switching and morphological changes influenced by hypoxia signaling, transforming growth factor-β, cyclic GMP modulation, and disrupted pericyte-endothelial communication. Recent single-cell RNA sequencing enabled the identification of a novel and specific pericyte marker, Higd1b , thereby improving pericyte identification and revealing novel pericyte subtypes. In this review, we summarize historical and recent insights into pericyte morphology and function, their increasingly recognized role in pulmonary hypertension pathobiology, and the potential to unlock novel therapeutic avenues targeting pericytes.

The earliest hematopoietic stem and progenitor cells (HSPCs) are generated from the ventral wall of the dorsal aorta, through endothelial-to-hematopoietic transition during vertebrate embryogenesis. Notch signaling is crucial for HSPC generation across vertebrates; however, the precise control of Notch during this process remains unclear. In the present study, we used multi-omics approaches together with functional assays to assess global DNA methylome dynamics during the endothelial cells to HSPCs transition in zebrafish, and determined that DNA methyltransferase 1 (Dnmt1) is essential for HSPC generation via repression of Notch signaling. Depletion of dnmt1 resulted in decreased DNA methylation levels and impaired HSPC production. Mechanistically, we found that loss of dnmt1 induced hypomethylation of Notch genes and consequently elevated Notch activity in hemogenic endothelial cells, thereby repressing the generation of HSPCs. This finding deepens our understanding of HSPC specification in vivo, which will provide helpful insights for designing new strategies for HSPC generation in vitro.