Mechanical and Thermodynamic Properties of Mesenchymal Stem Cells Differentiated into Myofibroblasts: A Commentary on the Article “Statistical Mechanics of Non-Muscle Myosin IIA in Human Bone Marrow-Derived Mesenchymal Stromal Cells Seeded in a Collagen Scaffold: A Thermodynamic Near-Equilibrium Linear System Modified by the Tripeptide Arg-Gly-Asp (RGD)”

Mesenchymal Stem Cells (MSCs) are multipotent stromal cells found in bone marrow and have the capacity to differentiate into myofibroblast. In contractile myofibroblasts, the molecular motor is the non-muscle myosin (NMIIA) which differs from the muscle myosin by its ultra-slow kinetics. The differentiation of MSCs into myofibroblasts is promoted by the “Transforming Growth Factor” (TGF-b) which represents a potentially target against tissue fibrosis and cancer. Myofibroblasts have been discovered by Gabbiani et al. [1,2] and are physiologically found in certain organs such as human placenta. They also play a key role in inflammatory and fibrotic processes and cancer stroma. They exhibit contractile properties qualitatively similar to those observed in muscles. However, quantitatively they are strongly different. Thus, their mechanical and thermodynamic properties are dramatically low compared to what is observed in skeletal or smooth muscles.

Multipotent Mesenchymal Stem Cells (MSCs) are found in bone marrow and in other tissues including cord blood, peripheral blood, and fetal lung and liver. They can differentiate into various cell types, including adipocytes, chondrocytes, osteoblasts, and myocytes. MSCs have a great capacity for self-renewal while maintaining their multipotency. They play a key role in making and repairing skeletal tissues, bone, cartilage and fat. MSCs can also undergo a myofibroblast differentiation, conjointly with the production of alpha Smooth Muscle Actin (α-SMA) in response to stimulation by means of the transforming growth factor-β (TGF-β). Myofibroblasts also play a fundamental role in tissue repair processes, inflammatory processes, fibrosis (heart, lung, kidney, liver, etc.) and cancer stroma. 

The presence of myofibroblasts in either physiological or pathological tissues gives them surprising contractile properties. These tissues are not muscles but present contractile properties qualitatively similar to those of muscles. These “non-muscles” contain numerous myofibroblasts [1,2]. Their molecular motor is the non-muscle myosin type IIA (NMIIA), whose the main characteristic is the dramatic slowness of molecular kinetics. Myofibroblasts are present in physiological tissues (human placenta) [3], in pathological tissues (fibrosis, inflamed tissues and cancer stroma [4,5] and in engineered tissues [6].  Although mechanical and thermodynamic properties of striated (skeletal and cardiac) and smooth muscles have been thoroughly described for a long time, those of non-muscle tissues remain relatively poorly reported. Similarly to muscles, non-muscle contractile tissues present four mechanical properties: i) they contract after electrical stimulation or in presence of KCl; ii) they relax by either decreasing the intracellular calcium concentration (isosorbide dinitrate (ISDN), Sildenafil) or inhibiting actin-myosin interactions; iii) they obey the Frank-Starling law [7,8], which specifies that when the initial length of contractile fibers increases, the developed tension increases; iv), they present the hyperbolic relationship between the maximum shortening velocity (V) and the level of isotonic tension (T) reported by Hill AV [9]. This property is of great importance because it allows to apply the phenomenological theory of muscle contraction due to A. Huxley [10] and to determine the molecular kinetics of actin-myosin Cross Bridges (CBs) [11]. The curvature of the T-V relationship can be introduced in Huxley’s equations [10] and makes it possible to determine the molecular properties of the molecular motor, namely: average individual force generated by a single actin-myosin interaction, number of active actin-myosin CBs, kinetics of attachment and detachment of CB, maximum myosin ATPase activity, total myosin content and maximum efficiency of the actin-myosin interaction.  In statistical mechanics, the huge number of myosin heads in muscles and non-muscle contractile issues makes it possible to apply the formalism of the grand canonical ensemble. This allows to determine the thermodynamic quantities namely: statistical entropy, internal energy, chemical affinity, thermodynamic force, thermodynamic flow and rate of entropy production. Normal striated and smooth muscles operate near equilibrium and in a linear stationary fashion. This means that the thermodynamic force linearly varies with the thermodynamic flow [12,13]. 

Besides muscles, non-muscle contractile tissues also exhibit contractile properties. One of these tissues is the human placenta, in which cells show mechanical similarities to smooth muscle cells [14]. The human term placenta presents an important source of pluripotent adult stem cells [15,16]. MSCs isolated from the placenta show similar characteristics to widely used bone marrow-derived MSCs [17]. Human placental villi are able to contract after stimulation with KCl [18-20]. Human placenta is a non-muscle contractile organ. It can contract due to the presence of numerous myofibroblasts contained in the placental villi [3]. The molecular motor is NMIIA [21]. Mechanical and thermodynamic properties of human placenta can be studied by using the same techniques than those used for muscle tissues [22-24]. They are qualitatively similar to those of muscles. Thus, contraction of the placental villi can be triggered by an electric field or after KCl exposure. Likewise, relaxation is induced in both contractile systems by the reuptake of calcium by the sarco-endoplasmic reticulum or by CB inhibition. The Frank-Starling phenomenon and the hyperbolic relation T-V are also observed. However, the molecular characteristics of NMIIA are quite different from those of muscles. The major mechanical difference between muscles and non-muscle contractile tissues is that non-muscle contractile tissues present mechanical properties dramatically slow compared with those of muscle myosins and develop less tension. In addition, molecular kinetics of NMIIA and maximum ATPase activity are extremely low compared with than those of muscle myosins. However, the force developed by a single actin-myosin interaction and the thermodynamic efficiency of the CB interaction are of same order of magnitude as that determined in both muscles and non-muscle contractile systems [13].

 There are other non-muscular contractile systems besides the placenta. We recently showed that Mesenchymal Stem Cells (MSCs) derived from human bone marrow taken from femoral heads obtained after fractures of the femoral neck and included in collagen scaffolds in the presence of TGF-b1, exhibited the same mechanical and thermodynamic properties as those of the human placenta [6]. Under these conditions, MSCs differentiate into myofibroblasts. Mechanical and thermodynamic properties of MSCs included in collagen scaffolds are similar to those of the human placenta. Like human placenta, they contract after electrical stimulation or KCl exposure, they relax under ISDN or Sildenafil, they present the Frank-Starling law and the T-V hyperbolic relationship [25,26]. The differentiation of fibroblasts into myofibroblasts is largely regulated by TGF-b1. TGF-b1 upregulates the canonical Wnt/b-catenin pathway positively, and downregulates the peroxisome proliferator activated receptor gamma (PPARg). The Wnt/b-catenin signaling promotes tissue fibrosis while PPARg prevents it [4]. Thus, TGF-b1 promotes the onset of fibrosis in the heart, lungs, liver and kidneys. Conversely, PPARg opposes the canonical Wnt signaling and tends to limit the processes of fibrosis. Myofibroblasts have been found in many fibrous diseases such as systemic sclerosis, idiopathic pulmonary fibrosis, cirrhosis of the liver, heart failure and myocardial infarction. They are also present in cancer stroma [27], retinal detachment and anterior capsular cataract. Chronic inflammatory lesions cause prolonged activation of fibroblasts which then differentiate into myofibroblasts. These can persist after a skin wound has closed, leading to a hypertrophic scar, especially in burns [28]. PPARg agonists can interrupt or prevent the profibrotic effects of TGF-β1, and the differentiation of fibroblasts into myofibroblasts. These mechanisms potentially represent interesting therapeutic targets in the fight against cancer and fibrosis.

Non-muscle contractile tissues containing a sufficient population of myofibroblasts acquire contractile properties qualitatively similar to those of muscles. However, their NMIIA molecular motor exhibits ultra-slow kinetics but develops elemental force of same order of magnitude compared with those of muscle myosins. Thermodynamically, the non-muscle contractile tissues operate near equilibrium in a stationary linear mode. The central role of myofibroblasts [29] reflects the interest of better understanding the contractile mechanisms of non-muscle tissues involved in tissue repair processes [30] but also in fibrosis [31] and cancers [32].