The big five in fibrosis: Macrophages, myofibroblasts, matrix, mechanics, and miscommunication

Intestinal fibrosis

Extensive tissue fibrosis is the end-stage process of a number of chronic conditions affecting the gastrointestinal tract, including inflammatory bowel disease (Crohn's disease, ulcerative colitis), ulcerative jejunoileitis, and radiation enteritis. Fibrogenesis is a physiological, reparative process that may become harmful as a consequence of the persistence of a noxious agent, after an excessive duration of the healing process. In this case, after replacement of dead or injured cells, fibrogenesis continues to substitute normal parenchymal tissue with fibrous connective tissue, leading to uncontrolled scar formation and, ultimately, permanent organ damage, loss of function, and/or strictures. Several mechanisms have been implicated in sustaining the fibrogenic process. Despite their obvious etiological and clinical distinctions, most of the above-mentioned fibrotic disorders have in common a persistent inflammatory stimulus which sustains the production of growth factors, proteolytic enzymes, and pro-fibrogenic cytokines that activate both non-immune (i.e., myofibroblasts, fibroblasts) and immune (i.e., monocytes, macrophages, T-cells) cells, the interactions of which are crucial in the progressive tissue remodeling and destroy. Here we summarize the current status of knowledge regarding the mechanisms implicated in gut fibrosis with a clinical approach, also focusing on possible targets of antifibrogenic therapies.

Development of photosynthetic sutures for the local delivery of oxygen and recombinant growth factors in wounds

Surgical sutures represent the gold standard for wound closure, however, their main purpose is still limited to a mechanical function rather than playing a bioactive role. Since oxygen and pro-regenerative growth factors have been broadly described as key players for the healing process, in this study we evaluated the feasibility of generating photosynthetic sutures that, in addition to mechanical fixation, could locally and stably release oxygen and recombinant human growth factors (VEGF, PDGF-BB, or SDF-1α) at the wound site. Here, photosynthetic genetically modified microalgae were seeded in commercially available sutures and their distribution and proliferation capacity was evaluated. Additionally, the mechanical properties of seeded sutures were compared to unseeded controls that showed no significant differences. Oxygen production, as well as recombinant growth factor release was quantified in vitro over time, and confirmed that photosynthetic sutures are indeed a feasible approach for the local delivery of bioactive molecules. Finally, photosynthetic sutures were tested in order to evaluate their resistance to mechanical stress and freezing. Significant stability was observed in both conditions, and the feasibility of their use in the clinical practice was therefore confirmed. Our results suggest that photosynthetic gene therapy could be used to produce a new generation of bioactive sutures with improved healing capacities. STATEMENT OF SIGNIFICANCE: Disruption of the vascular network is intrinsic to trauma and surgery, and consequently, wound healing is characterized by diminished levels of blood perfusion. Among all the blood components, oxygen and pro-regenerative growth factors have been broadly described as key players for the healing process. Therefore, in this study we evaluated the feasibility of generating photosynthetic sutures that, in addition to mechanical fixation, could locally and stably release oxygen and recombinant human growth factors at the wound site. This novel concept has never been explored before for this type of material and represents the first attempt to create a new generation of bioactive sutures with improved regenerative capabilities.

Platelet-Rich Plasma Prevents In Vitro Transforming Growth Factor-β1-Induced Fibroblast to Myofibroblast Transition: Involvement of Vascular Endothelial Growth Factor (VEGF)-A/VEGF Receptor-1-Mediated Signaling †

The antifibrotic potential of platelet-rich plasma (PRP) is controversial. This study examined the effects of PRP on in vitro transforming growth factor (TGF)-β1-induced differentiation of fibroblasts into myofibroblasts, the main drivers of fibrosis, and the involvement of vascular endothelial growth factor (VEGF)-A in mediating PRP-induced responses. The impact of PRP alone on fibroblast differentiation was also assessed. Myofibroblastic phenotype was evaluated by confocal fluorescence microscopy and western blotting analyses of α-smooth muscle actin (sma) and type-1 collagen expression, vinculin-rich focal adhesion clustering, and stress fiber assembly. Notch-1, connexin 43, and VEGF-A expression were also analyzed by RT-PCR. PRP negatively regulated fibroblast-myofibroblast transition via VEGF-A/VEGF receptor (VEGFR)-1-mediated inhibition of TGF-β1/Smad3 signaling. Indeed TGF-β1/PRP co-treated fibroblasts showed a robust attenuation of the myofibroblastic phenotype concomitant with a decrease of Smad3 expression levels. The VEGFR-1 inhibition by KRN633 or blocking antibodies, or VEGF-A neutralization in these cells prevented the PRP-promoted effects. Moreover PRP abrogated the TGF-β1-induced reduction of VEGF-A and VEGFR-1 cell expression. The role of VEGF-A signaling in counteracting myofibroblast generation was confirmed by cell treatment with soluble VEGF-A. PRP as single treatment did not induce fibroblast myodifferentiation. This study provides new insights into cellular and molecular mechanisms underpinning PRP antifibrotic action.

Signaling Mechanisms of Myofibroblastic Activation: Outside-in and Inside-Out

Myofibroblasts are central mediators of fibrosis. Typically derived from resident fibroblasts, myofibroblasts represent a heterogeneous population of cells that are principally defined by acquired contractile function and high synthetic ability to produce extracellular matrix (ECM). Current literature sheds new light on the critical role of ECM signaling coupled with mechanotransduction in driving myofibroblastic activation. In particular, transforming growth factor β1 (TGF-β1) and extra domain A containing fibronectin (EDA-FN) are thought to be the primary ECM signaling mediators that form and also induce positive feedback loops. The outside-in and inside-out signaling circuits are transmitted and integrated by TGF-β receptors and integrins at the cell membrane, ultimately perpetuating the abundance and activities of TGF-β1 and EDA-FN in the ECM. In this review, we highlight these conceptual advances in understanding myofibroblastic activation, in hope of revealing its therapeutic anti-fibrotic implications.

Matrix molecules and skin biology

An extracellular matrix (ECM) is a prerequisite for multicellular life. It is adapted to tissues and constantly undergoes changes to preserve microenvironmental homeostasis. The ECM acts as a structural scaffold that establishes tissue architecture and provides tensile strength. It has cell-instructive functions by serving as a reservoir and presenter of soluble agents, being directly signaling, integrating transmission of mechanical and biological cues, or serving as a co-factor potentiating signaling. The skin contains a highly developed, mechanically tough, but yet flexible ECM. The tissue-specific features of this ECM are largely attributed by minor ECM components. A large number of genetic and acquired ECM diseases with skin manifestations, provide an illustrative testament to the importance of correct assembly of the ECM for dermal homeostasis. Here, we will present the composition and features of the skin ECM during homeostasis and regeneration. We will discuss genetic and acquired ECM diseases affecting skin, and provide a short outlook to therapeutic strategies for them.

The journey of cells through regeneration

The process of building an organ, appendage, or organism requires the precise coordination of cells in space and time. Regeneration of those same tissues adds an additional element of complexity, emerging from the chaos of disease or injury to build a mass of progenitors from mature tissue. Translating insights from natural examples of tissue regeneration into engineered regenerative therapies requires a deep understanding of the journey of a cell directly following injury to its contribution to functional, scaled replacement tissue. Here we step through the chronological phases of regeneration and highlight emerging work that brings us closer to elucidating the unique intrinsic and extrinsic properties of cells during epimorphic regeneration.

Exosomes and exosomal miRNAs from muscle-derived fibroblasts promote skeletal muscle fibrosis

Exosomes, natural carriers of mRNAs, non-coding RNAs and proteins between donor and recipient cells, actively contribute to cell-cell communication. We investigated the potential pro-fibrotic role of exosomes released by muscle-derived fibroblasts of Duchenne muscular dystrophy (DMD) patients, and of miRNAs carried by exosomes. By fibrosis focused array analysis we found that exosomes from DMD fibroblasts, had significantly higher levels of miR-199a-5p, a miRNA up-regulated in fibrotic conditions, compared to control exosomes, while levels in myoblast-derived exosomes were not increased. In control fibroblasts, exposure to DMD fibroblast-derived exosomes induced a myofibroblastic phenotype with increase in α-smooth actin, collagen and fibronectin transcript and protein expression, soluble collagen production and deposition, cell proliferation, and activation of Akt and ERK signaling, while exposure to control exosomes did not. Transfecting control fibroblasts or loading control exosomes with miR-199a-5p mimic or inhibitor induced opposing effects on fibrosis-related mRNAs and proteins, on collagen production and Akt and ERK pathways. Finally, injection of DMD fibroblast-derived exosomes into mouse tibialis anterior muscle after cardiotoxin-induced necrosis, produced greater fibrosis than control exosomes. Our findings indicate that exosomes produced by local fibroblasts in the DMD muscle are able to induce phenotypic conversion of normal fibroblasts to myofibroblasts thereby increasing the fibrotic response. This conversion is related to transfer of high levels of miR-199a-5p and to reduction of its target caveolin-1; both, therefore, are potential therapeutic targets in muscle fibrosis.

Organ and tissue fibrosis: Molecular signals, cellular mechanisms and translational implications

Fibrosis denotes excessive scarring, which exceeds the normal wound healing response to injury in many tissues. Although the extracellular matrix deposition appears unstructured disrupting the normal tissue architecture and subsequently impairing proper organ function, fibrogenesis is a highly orchestrated process determined by defined sequences of molecular signals and cellular response mechanisms. Persistent injury and parenchymal cell death provokes tissue inflammation, macrophage activation and immune cell infiltration. The release of biologically highly active soluble mediators (alarmins, cytokines, chemokines) lead to the local activation of collagen producing mesenchymal cells such as pericytes, myofibroblasts or Gli1 positive mesenchymal stem cell-like cells, to a transition of various cell types into myofibroblasts as well as to the recruitment of fibroblast precursors. Clinical observations and experimental models highlighted that fibrosis is not a one-way road. Specific mechanistic principles of fibrosis regression involve the resolution of chronic tissue injury, the shift of inflammatory processes towards recovery, deactivation of myofibroblasts and finally fibrolysis of excess matrix scaffold. The thorough understanding of common principles of fibrogenic molecular signals and cellular mechanisms in various organs - such as liver, kidney, lung, heart or skin - is the basis for developing improved diagnostics including biomarkers or imaging techniques and novel antifibrotic therapeutics.

Cellular and molecular mechanisms of kidney fibrosis

Renal fibrosis is the final pathological process common to any ongoing, chronic kidney injury or maladaptive repair. It is considered as the underlying pathological process of chronic kidney disease (CKD), which affects more than 10% of world population and for which treatment options are limited. Renal fibrosis is defined by excessive deposition of extracellular matrix, which disrupts and replaces the functional parenchyma that leads to organ failure. Kidney's histological structure can be divided into three main compartments, all of which can be affected by fibrosis, specifically termed glomerulosclerosis in glomeruli, interstitial fibrosis in tubulointerstitium and arteriosclerosis and perivascular fibrosis in vasculature. In this review, we summarized the different appearance, cellular origin and major emerging processes and mediators of fibrosis in each compartment. We also depicted and discussed the challenges in translation of anti-fibrotic treatment to clinical practice and discuss possible solutions and future directions.

Decoding fibrosis: Mechanisms and translational aspects

Fibrosis, a complex process of abnormal tissue healing which inevitably leads to loss of physiological organ structure and function, is a worldwide leading cause of death. Despite a large body of research over the last two decades, antifibrotic approaches are mainly limited to organ replacement therapy generating high costs of medical care. In this translational issue, a unique group of basic and clinical researchers provide meaningful answers to a desperate call of society for effective antifibrotic treatments. Fortunately, a plethora of novel fibrogenic factors and biomarkers has been identified. Noninvasive diagnostic methods and drug delivery systems have been recently developed for the management of fibrosis. Consequently, a large number of exciting clinical trials addressing comprehensive, organ and stage-specific mechanisms of fibrogenesis are ongoing. By critically addressing previously unsuccessful and novel promising therapeutic strategies, we aim to spread hope for future treatments of the various forms of organ fibrosis.

New developments on skin fibrosis - Essential signals emanating from the extracellular matrix for the control of myofibroblasts

Many different diseases are associated with fibrosis of the skin. The clinical symptoms can vary considerably with a broad range from isolated small areas to the involvement of the entire integument. Fibrosis is triggered by a multitude of different stimuli leading to activation of the immune and vascular system that then initiate fibroblast activation and formation of matrix depositing and remodeling myofibroblasts. Ultimately, myofibroblasts deposit excessive amounts of extracellular matrix with a pathological architecture and alterations in growth factor binding and biomechanical properties, which culminates in skin hardening and loss of mobility. Treatment depends certainly on the specific type and cause of the disease, for the autoimmune driven localized and systemic scleroderma therapeutic options are still limited, but recent research has pointed out diverse molecular targets and mechanisms that can be exploited for the development of novel antifibrotic therapy.