Cardiac Fibroblasts: Helping or Hurting

Abstract

Cardiac fibroblasts (CFs) are the essential cell type for heart morphogenesis and homeostasis. In addition to maintaining the structural integrity of the heart tissue, muscle fibroblasts are involved in complex signaling cascades that regulate cardiomyocyte proliferation, migration, and maturation. While CFs serve as the primary source of extracellular matrix proteins (ECM), tissue repair, and paracrine signaling, they are also responsible for adverse pathological changes associated with cardiovascular disease. Following activation, fibroblasts produce excessive ECM components that ultimately lead to fibrosis and cardiac dysfunction. Decades of research have led to a much deeper understanding of the role of CFs in cardiogenesis. Recent studies using the single-cell genomic approach have focused on advancing the role of CFs in cellular interactions, and the mechanistic implications involved during cardiovascular development and disease. Arguably, the unique role of fibroblasts in development, tissue repair, and disease progression categorizes them into the friend or foe category. This brief review summarizes the current understanding of cardiac fibroblast biology and discusses the key findings in the context of development and pathophysiological conditions.

Keywords: cardiac fibrosis; cardiac muscle; cardiomyocytes; cardiomyopathy; fibroblasts; signaling.

Figure 1

Cardiac fibroblast origin and function. CFs are crucial structural cells for the proper development of heart tissue. During embryonic development, the proepicardium organ gives rise to the epicardium-derived fibroblast progenitor cells through a process called epithelial–mesenchymal transition (EMT), and through endothelial by an endothelial–mesenchymal transformation (EndMT) process they are transitioned into cardiac fibroblast. CFs are characterized by the expression of unique markers such as CD90, DDR2, Sca1, FSP1, Fibronectin, Vimentin, Collagen Type I, and Collagen Type III. Cardiac fibroblasts play several key roles in maintaining tissue structure, function, and repair. One of the major roles of CF is to provide structural support to the developing organ. CFs synthesize the ECM by depositing collagen fiber, proteoglycans, elastin, fibronectin proteins, and laminins, and remodel the ECM by covalent crosslinking, protein glycosylation, as well as by secretion of matrix metalloproteinases (MMPs). Interactions between CM-CFs have led to increased cardiomyocyte maturation. Further, CFs play an important role in paracrine signaling by secreting a number of chemical signals including cytokines (TNFα, IFNγ, IL-6), chemokines (MMPs), and growth factors. Multiple studies have shown their role in immune regulation and inflammation following injury.

Figure 2

Cardiac fibroblast activation in response to injury. Resident fibroblasts are integral components of healthy tissues, actively participating in maintaining tissue structure, function, and homeostasis. Upon mechanical stress or tissue injuries, TGF-β and AngII signaling pathways are activated, activating resting fibroblast into myofibroblast. Myofibroblasts are specialized cells with high αSMA expression. These cells increased the production of ECM proteins such as collagen, fibronectin, and other microfibrillar proteins. Myofibroblasts play a crucial role in tissue repair but can also induce pathological fibrosis when their activation is dysregulated.

Figure 3

Signaling pathways in cardiac fibroblast. (A) Activated PDGFR stimulates the Ras pathway, which leads to the activation of RAF proto-oncogene serine/threonine protein kinase (Raf-1). Raf-1 activates downstream effectors, including dual-specificity mitogen-activated protein kinase (MEK) and extracellular signal-regulated protein kinase (ERK1/2) which promote the phosphorylation of several transcription factors to induce cell proliferation, cell survival, and ECM-related gene expression. (B) TGF-β signaling pathway. TGF-β homodimers phosphorylate and form a heterotetrameric complex with two type I receptors. The signaling domain of the type I receptor mediates phosphorylation and the activation of Smad proteins. The activated Smad complex forms a transcriptional module with several transcription factors, and co-factors to promote the transcription of ECM-related target genes. TGF-β activated the AKT pathway via a RhoA-dependent manner to induce the transcription of target genes. (C) Wnt signaling pathway. Wnt ligands bind to a frizzled receptor and co-receptor LRP. In the absence of a Wnt signal, cytoplasmic b-catenin phosphorylated by a multiprotein complex consisting of Axin, APC protein, and several kinases, is targeted for ubiquitin-mediated degradation. Upon Wnt binding, the destruction complex is dissembled, allowing β-catenin to translocate into the nucleus, where it binds with transcription factors (TCF/LEF) to regulate downstream gene expression. (D) Hippo signaling pathway. Various physiological or pathological signals induce the Hippo signaling pathway in fibroblasts. Upon activation, Hippo kinase complexes MST1/2, MOB1 SAV, and LATS1/2 induce phosphorylation of YAP and TAZ. Phosphorylated YAP and TAZ are sequestered in the cytoplasm by 14-3-3 proteins or targeted for degradation. The inactivation of the Hippo kinase complex results in the nuclear translocation of YAP and TAZ. In the nucleus, YAP and TAZ bind with TEAD factors to regulate gene expression.

Figure 4

Modifying cardiac fibroblasts for diverse applications. Although detrimental to the injured heart tissue, recent studies have provided ways to utilize CFs for beneficial purposes. Direct reprogramming approaches have been tested using cardiogenic factors (Gata4, Hand2, Tbx5, myocardin) to cardiomyocytes both in vitro and in vivo. These methods have shown to reduced scarring with improved cardiac function. In the in vivo reprogramming, CFs differentiate into cardiomyocytes and mature to restore the damaged heart function [230]. Another strategy to utilize CF is to reprogram them to pluripotent stem cells using Yamanaka factors. These strategies have revolutionized cardiac regenerative medicine as well as developed cardiac disease models. In recent years, the reprogrammed CFs are being tested for personalized medicine. Cardiac fibroblast is also modified with modifiedRNAs (modRNA) or CAR-T cells to repair damaged hearts. Recently, CAR-T cell therapy has been used for cardiac fibrosis treatment [231]. Fibroblast activation protein (FAP), a glycoprotein expressed on a fibroblast surface during cardiac injury or fibrosis, are the target for the CAR-T cell approach to reduce cardiac fibrosis [232,233]. MicroRNA (miRNA) based therapy also modulates the cardiac fibroblast function to repair/regenerate the damaged heart [234,235]. In this approach, a combination of different miRNAs were used to regulate the proliferation and migration ability of cardiac fibroblast as well as activation of fibroblast into myofibroblast to repair the cardiac injury [236]. modRNA technology also targets specifically CFs to treat cardiac diseases [237]. Mice models of myocardial infarction showed that the cocktail of 7G modRNA treatment reduced cardiac scars as compared to the control and also reduced the collagen and fibronectin expression in treated mice [238].