Background/Purpose: Systemic sclerosis (SSc) is a devastating autoimmune disease characterized by excessive production and accumulation of extracellular matrix molecules leading to fibrosis of skin and other internal organs. However, the mechanisms and the main cellular participants in SSc skin fibrosis remain incompletely understood. Here, we sought to provide a detailed and comprehensive characterization of the cellular and molecular events underlying skin fibrosis in SSc.

Methods: To understand the unbiased cellular composition and cell states of healthy normal skin (NS) and (SSc) skin, we generated single cell suspensions of skin biopsies from 18 NS donors and 22 SSc patients and performed single cell RNA sequencing (scRNA-seq). To localize the major cell types detected by scRNA-seq in SSc skin, we performed spatial sequencing (spatial-seq) on the SSc skin sample using the 10X Visium platform. The R package Seurat was used to analyze and integrate the scRNA-seq and spatial-seq datasets. The R package Monocle was used to perform pseudotime trajectory analysis. CellphoneDB was used to construct ligand-receptor networks among different cell types. Fibroblasts and endothelial cells isolated from dcSSc skin were cultured with TRULI and verteporfin to test the roles of Hippo pathway effectors in myofibroblast differentiation and EndoMT.

Results: We demonstrated a dual source of extracellular matrix deposition in SSc skin from both myofibroblasts and endothelial to mesenchymal transitioning cells (EndoMTs) (Figure 1). Importantly, pseudotime trajectory analysis, which aligns the relationship of fibroblast and endothelial subsets identified a central role of Hippo pathway effectors in promoting and maintaining myofibroblast differentiation and EndoMT (Figure 2). Ligand-receptor analysis revealed that myofibroblasts and EndoMTs act as central communication hubs that drive key pro-fibrotic signaling pathways in SSc (Figure 3). Inhibition of TEAD/YAP interactions, key transcription coactivators in Hippo signaling, reversed pro-fibrotic phenotypes in myofibroblasts and EndoMTs. Together, these data provide comprehensive and detailed characterization of myofibroblast differentiation and EndoMT in SSc skin and identify a novel pathway to target for fibrosis reversal.

Conclusion: Hippo pathway effectors, which regulate cell proliferation, inflammatory, and apoptotic pathways, promote and maintain myofibroblast differentiation and endothelial-to-mesenchymal transition in systemic sclerosis. Modulation of the Hippo pathway should be further studied in SSc.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/hippo-pathway-effectors-promote-and-maintain-myofibroblast-differentiation-and-endothelial-to-mesenchymal-transition-in-systemic-sclerosis/