Fibroblasts in Diabetic Foot Ulcers

Abstract

Fibroblasts are stromal cells ubiquitously distributed in the body of nearly every organ tissue. These cells were previously considered to be "passive cells", solely responsible for ensuring the turnover of the extracellular matrix (ECM). However, their versatility, including their ability to switch phenotypes in response to tissue injury and dynamic activity in the maintenance of tissue specific homeostasis and integrity have been recently revealed by the innovation of technological tools such as genetically modified mouse models and single cell analysis. These highly plastic and heterogeneous cells equipped with multifaceted functions including the regulation of angiogenesis, inflammation as well as their innate stemness characteristics, play a central role in the delicately regulated process of wound healing. Fibroblast dysregulation underlies many chronic conditions, including cardiovascular diseases, cancer, inflammatory diseases, and diabetes mellitus (DM), which represent the current major causes of morbidity and mortality worldwide. Diabetic foot ulcer (DFU), one of the most severe complications of DM affects 40 to 60 million people. Chronic non-healing DFU wounds expose patients to substantial sequelae including infections, gangrene, amputation, and death. A complete understanding of the pathophysiology of DFU and targeting pathways involved in the dysregulation of fibroblasts are required for the development of innovative new therapeutic treatments, critically needed for these patients.

Keywords: chronic inflammation; chronic wound; diabetic foot ulcer; diabetic wound; fibroblast; myofibroblast.

Figure 1

Heterogeneity of the dermal fibroblast population. Mouse dermal fibroblasts exhibit variations on the cellular and molecular levels based on their distribution in the dermis. The two main subpopulations are (a) papillary fibroblasts in the most superficial layer of the dermis and (b) reticular fibroblasts in the deeper layer of the dermis. These fibroblasts respond to different signaling pathways (simplified in figure) such as Wnt and Notch pathways in the papillary fibroblasts vs. TGF-β pathway in the reticular fibroblasts. They are characterized by differences in their phenotype with distinct morphology, surface markers expression, genetic signature, and epigenetic modifications. Similar markers were also characterized in the human papillary and reticular fibroblast populations. APC: Adenomatous Polyposis Coli; APCs: Antigen Presenting Cells; CSL: C-promoter binding factor 1 (CBF1), suppressor of hairless (Su(H)), lin-12 and glp1 (Lag1); Co-A: Co-enzyme -A; Col1A1: Collagen type I alpha 1; DLK1: Delta-like non-canonical Notch ligand 1; Fbn1: Fibrillin 1; Fgf-7: Fibroblast Growth Factor -7; FSP1: Fibroblast-specific protein 1; GSK: Glycogen synthase kinase; Itga8: Alpha-8 integrin; LRIG1: Leucine-rich repeats and immunoglobulin-like domains 1; MEK: Mitogen-activated extracellular signal-regulated kinase; NOS: Nitric oxide synthase; TCF/LEF: T-cell factor/lymphoid enhancer factor; TGF-β: Transforming Growth Factor-β; Wnt: Wingless-related integration site.

Figure 2

Fibroblasts plasticity during wound healing. Upon injury, fibroblasts exhibit plasticity by trans-differentiation into myofibroblasts that will drive the next phase of the healing process. This activation is mainly regulated by the TGF-β signaling pathway. Epigenetic modifications such as DNA methylation also modulate the mechanism of fibroblasts trans-differentiation. Myofibroblasts’ ability to contract favors closure of the wound. Fibroblasts also can also trans-differentiate into adipocytes during the wound healing process. Myofibroblasts are later cleared from the wound after dedifferentiation into fibroblasts that are later cleared by apoptosis and phagocytosis by macrophages during the wound healing process. TGF-β: Transforming Growth Factor-β.

Figure 3

Fibroblasts’ pivotal role in the different phases of acute/non-impaired wound healing vs. chronic diabetic wound healing. Fibroblasts in (a) acute/non-impaired wound healing are activated and trans-differentiate into myofibroblasts upon injury responding to hypoxia, after stimulation of the immune system in the inflammation phase by inflammatory cytokines and factors. TGF-β signaling pathway is preferentially activated, regulating the trans-differentiation phase of fibroblasts. Myofibroblasts play a pivotal role in the following phases, secreting additional regulatory growth factors, cytokines and chemokines, stimulating proliferation of additional myofibroblasts, keratinocytes, and endothelial cells, and promoting angiogenesis. Myofibroblasts generate MMPs to break down the old ECM and generate new ECM during the remodeling phase and dedifferentiate into fibroblasts for clearance by macrophage phagocytosis. Fibroblasts in (b) chronic diabetic wound healing are impaired by additional ROS, AGEs, and LPSs that overstimulate the immune system, prolonging the inflammatory phase which is further prolonged by preferential activation of Notch signaling that prevents the trans-differentiation of fibroblasts into myofibroblasts. Consecutive phases of proliferation, remodeling, and clearance are consequently altered. AGE: advanced glycation end product; ECM: extracellular matrix; LPSs: lipopolysaccharides. MMPs: matrix metalloproteinases; NOS: nitric oxide synthase; ROS: reactive oxygen species.