Myofiboblast

Myofibroblasts are modified fibroblasts. Myofibroblasts were first described in 1971 by Gabbiani and coworkers in rat granulation tissue as cells that exhibit morphological characteristics of "conventional tissue fibroblasts" and contractile smooth muscle cells in that they are "capable of active spasm" (hence the name, although "spasmoblast" has also been discussed) (Gabbiani et al. 1971). Myofibroblasts combine the properties of fibroblasts and smooth muscle cells and have a significantly higher contractile capacity than normal fibroblasts. In contrast to smooth muscle cells, myofibroblasts are not surrounded by a basal lamina. Myofibroblasts are able to produce endogenous collagen.

Myofibroblasts also occur as pericytes in the capillary walls, in the cortical stroma of the ovary and in the tunica albuginea of the testis. Myofibroblasts have elongated cell bodies and numerous cytoplasmic projections, which sometimes give the cells a star-shaped appearance. They are in contact with neighboring cells via adherens junctions and gap junctions. The intracellular microfilaments are aligned parallel to the longitudinal axis of the cell and show dense bodies. They are connected to the abundant extracellular matrix via cell-stroma contact points. There is continuity between intracellular microfilaments and extracellular fibronectin fibers via a transmembrane complex (fibronexus). The cell nucleus of the myofibroblasts shows deep indentations.

During wound healing, myofibroblasts usually develop from fibroblasts that are stimulated to cell division by activated macrophages. The resulting myofibroblasts migrate with the macrophages into the wound area. The production of collagen restores the integrity of the tissue and leads to scar formation. Reduced activation of the myofibroblasts leads to insufficient wound healing. Myofibroblasts are directly linked to tissue repair and fibrosis. In addition, excess collagen leads to reduced vascularity over time. This change in vascularity makes fibrotic regions more susceptible to loss of function, necrosis, tissue atrophy and a decrease in fibroblast numbers.
The transition from macrophages to myofibroblasts has been shown to contribute to interstitial fibrosis in chronic renal transplant injury. Furthermore, the transition of bone marrow-derived M2 macrophages to myofibroblasts in the kidney graft is regulated by a Smad3-dependent mechanism (Wang YY et al. 2017).
Myofibroblasts play an important physiological role in the rapid repair of injured tissue by scar formation, e.g. in the skin after trauma or in the heart after a myocardial infarction. In addition, myofibroblasts are important effector cells in almost all organ fibrosis by accumulating scar tissue beyond normal repair. Conventional fibroblasts form myofibroblasts in all injured or fibrotic organs, e.g., in the liver, heart, as cancer-associated fibroblasts (CAFs) around tumors, and in dermal fibroblast lineages in the skin (Jiang and Rinkevich 2020). Pericytes have been identified as another major component of the myofibroblast pool in lung, kidney, skin, muscle and liver (Affo et al. 2017). It is likely that myofibroblasts can arise from multiple precursors, depending on the organ, impairment, timing and type of activation signal. Extracellular signals that regulate myofibroblast activation and activity may originate from other cell types and the extracellular matrix (ECM) of the healing and fibrotic environment. Myofibroblast activation is promoted by chemical and mechanical stimuli that form positive feedback loops that are central to the pathogenesis of fibrosis.

Myofibroblasts are physically prone to adherence and are poised to form "localized intercellular junctions" (Gabbiani et al. 1971), in addition to autocrine and paracrine exchange of soluble cytokines and growth factors, and production of exosomes and microvesicles (Zanotti et al. 2018). Gap junctions enable the exchange of ions and small molecules between myofibroblasts and fibroblasts, cardiomyocytes, mast cells and possibly also the endothelium (Lemoinne et al. 2015). Several cadherins are expressed in myofibroblast adhesion junctions, of which cadherin-11 (also known as OB-cadherin) is the most highly expressed and is co-regulated with myofibroblast activation (Valletta et al. 2012). Cadherin-11 mediates mechanoresistant connections between myofibroblasts and direct communication with macrophages. Cadherin-2 (also known as N-cadherin) transmits contractile force between myofibroblasts and cardiomyocytes, and heterotypic connections between cadherin-1 (also known as E-cadherin) expressing cancer cells and cadherin-2 expressing CAFs have been described (Labernadie et al. 2017). In addition, there are junctional adhesion molecules (JAMs) between myofibroblasts and endothelial cells and zonula occludens protein 1 (TJP1) in dermal fibroblasts. Finally, the cell adhesion molecules (CAMs) ICAM1 and VCAM1 are expressed in subtypes of myofibroblasts (Fontani et al. 2016), where they promote communication with immune cells, including T cells and neutrophils. In addition to direct intercellular communication, force transmission from myofibroblasts to the fibrous ECM generates mechanical signals. Other cells, such as macrophages, follow changes in ECM elongation rate, i.e. "pulling" events generated by contracting myofibroblasts, and cancer cells have been shown to follow ECM signals emanating from fibroblasts.

Activation states and characteristics: Myofibroblasts are gradually activated from their precursors at different stages, which has already been described in the seminal discovery. At the beginning of healing, myofibroblasts showed "the cytological structure considered 'typical' (i.e. numerous cisternae of rough endoplasmic reticulum and many mitochondria)" and later "bundles of packed fibrils resembling those of smooth muscle" (Gabbiani et al. 1971). Myofibroblast precursors express collagen type I proteins, platelet-derived growth factor(PDGF) alpha (PDGFRA) and/or receptor beta(PDGFRB) receptors, a marker for the mesenchymal lineage "hypermethylated in cancer 1" (Scott et al. 2019) and other markers in a similar manner in different organs.

Under continued mechanical stress and the addition of pro-fibrotic factors such as transforming growth factor beta-1 (TGFB1, hereafter TGF-β1) and the extradomain A (ED-A) splice variant of fibronectin (FN) ED-A FN, myofibroblasts are fully activated by neoexpression of α-Smooth Muscle Actin (α-SMA). Expression of alpha-SMA confers high contractile force to stress fibers and is the most commonly used myofibroblast marker. It is important to understand myofibroblast activation as a gradual maturation process and not to expect all features and functions in a single cell simultaneously. It is not known whether deactivated myofibroblasts revert to their respective precursor state, i.e. reassume pericyte, FAP or MSC properties, or whether they lose contractile properties and end up in a kind of cellular twilight state. Alternative myofibroblast states that contribute to their dissolution during normal tissue repair but are dysregulated in fibrosis are senescence (Merkt et al. 2020) or suicide by entering intrinsic or extrinsic apoptosis pathways (Hinz et al. 2019).

1. Affo, S et al (2017). The role of cancer-associated fibroblasts and fibrosis in liver cancer. Annu. Rev. Pathol 12: 153-186.
2. Fontani F et al. (2016) Tumor necrosis factor-alpha up-regulates ICAM-1 expression and release in intestinal myofibroblasts by redox-dependent and -independent mechanisms. J Cell Biochem 117: 370-381.
3. Gabbiani G et al. (1971) Presence of modified fibroblasts in granulation tissue and their possible role in wound contraction. Experientia 27: 549-550.
4. Hinz B (2006). Masters and servants of the force: the role of matrix adhesions in myofibroblast force perception and transmission. Eur J Cell Biol 85: 175-181.
5. Hinz B et al (2019). Mechanical regulation of myofibroblast phenoconversion and collagen contraction. Exp Cell Res 379: 119-128.
6. Jiang D et al (2020). Scars or regeneration?-dermal fibroblasts as drivers of diverse skin wound responses. Int J Mol Sci 21: 617.
7. Labernadie A et al (2017). A mechanically active heterotypic E-cadherin/N-cadherin adhesion enables fibroblasts to drive cancer cell invasion. Nat Cell Biol 19: 224-237.
8. Lemoinne S et al. (2015) Portal myofibroblasts promote vascular remodeling underlying cirrhosis formation through the release of microparticles. Hepatology 61: 1041-1055.
9. Merkt W et al (2020). Senotherapeutics: targeting senescence in idiopathic pulmonary fibrosis. Semin. Cell Dev Biol 101: 104-110.
10. Pakshir P et al. (2020) The myofibroblast at a glance. J Cell Sci 133:jcs227900.
11. Scott RW et al. (2019) Hic1 defines quiescent mesenchymal progenitor subpopulations with distinct functions and fates in skeletal muscle regeneration. Cell Stem Cell 25: 797-813.e9.
12. Valletta D et al. (2012). Expression and function of the atypical cadherin FAT1 in chronic liver disease. Biochem. Biophys Res Commun 426: 404-408.
13. Wang YY et al. (201q7) Macrophage-to-Myofibroblast Transition Contributes to Interstitial Fibrosis in Chronic Renal Allograft Injury. J Am Soc Nephrol 28:2053-2067.
14. Zanotti S et al (2018). Exosomes and exosomal miRNAs from muscle-derived fibroblasts promote skeletal muscle fibrosis. Matrix Biol 74: 77-100.