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May the Force Be with You
Developmental Cell
Spotlight May the Force Be with You Jiao Li1 and Nan Tang1,* 1National Institute of Biological Sciences, Beijing 102206, China *Correspondence: [email protected] https://doi.org/10.1016/j.devcel.2018.11.041
Elucidating the mechanical force-induced regulatory machineries is extremely important for further understanding of the mechanisms underlying development, regeneration, and disease. Recent work demonstrates that mechanical forces generated by blood flow in liver vessels function to regulate embryonic liver development and may also contribute to regeneration in adult liver. More than a century ago, the German anatomist and surgeon Julius Wolff boldly stated the view that bones can remodel themselves over time so that they become stronger as loading force increases. Consistent with this early idea, it is now known that a variety of mechanical forces, including shear forces caused by fluid viscosity, compression or traction forces exerted by surrounding cells, and even the rigidity of the extracellular matrix, are translated into variable biochemical signals in cells. Specific cellular, biochemical, and mechanical players are required to orchestrate physiology at the subcellular, cellular, tissue, organ, and whole-animal levels during embryonic development, homeostasis, and regeneration (Guillot and Lecuit, 2013; Petridou et al., 2017; Vining and Mooney, 2017). Identifying these players and understanding the complex crosstalk among them is essential for the elucidation of the mechanical force-induced regulatory machineries underlying elegantly sophisticated developmental processes. Reporting recently in Nature, Lorenz et al. (2018) demonstrated that mechanical forces generated by blood flow in liver vessels function to regulate embryonic liver development and, moreover, may also contribute to regeneration in the adult liver. The authors first noticed that murine embryonic livers expand substantially between embryonic days 12.5 (E12.5) and E13.5. Their careful observations also revealed that blood starts to perfuse into the blood vessels of the liver at E11.5 and does so in a highly spatially controlled manner: starting from the periphery and proceeding toward the center of the liver. These observations, viewed alongside their additional finding that proliferating hepatocytes are preferentially distributed at peripheral regions of livers at E11.5,
led the authors to propose that vascular perfusion may induce the proliferation of hepatocytes and thereby subsequently drive liver development and possibly even regeneration. It was previously appreciated that shear forces generated by dynamic blood flows can induce mechanical signals in endothelial cells. The authors therefore investigated whether any signaling events are activated specifically by blood perfusion in liver sinusoidal endothelial cells (LSECs). In light of the previous discovery that VEGFR3 activation in lymphatic endothelial cells requires sheer force-mediated activation of b1 integrin (ITGB1) (PlanasPaz et al., 2012), the authors investigated whether ITGB1 and VEGFR3 are also activated in LSECs in embryonic livers at E11.5 and indeed found that the activated forms of both ITGFB1 and VEGFR3 were conspicuously abundant in the peripheral liver regions that are characterized by both extensive blood perfusion and hyperproliferation of hepatocytes. Next, their loss-of-perfusion and gainof-perfusion experiments showed that the activation of both ITGB1 and VEGFR3 in LSECs was significantly reduced when blood perfusion was blocked by pharmacologically halting the embryo’s heartbeat. Conversely, increased ITGB1 and VEGFR3 activation occurred when blood perfusion was accelerated by increasing the embryo’s heart rate. The genetic evidence showing that impaired liver development occurs in LSEC Itgb1-deficient and Vegfr3-deficient mouse embryos collectively demonstrates that the mechanical force generated by blood perfusion does indeed activate ITGB1/VEGFR3/ HGF signaling in LSECs and consequently promotes embryonic liver development. The authors provide three lines of evidence suggesting that a similar mecha-
nism might occur during adult liver regrowth: (1) both ex vivo and in vitro mechanical stretching treatments activated ITGFB1/VEGF3/HGF signaling in adult hepatic ECs; (2) primary human hepatocytes cultured in medium collected from stretched human LSECs displayed increased proliferation and decreased apoptosis as compared to those in medium from non-stretched human LSECs; (3) in healthy adult humans, liver volume is positively correlated with the blood pressure. These facts collectively suggest that sheer-force-activated endothelial ITGFB1/VEGF3/HGF signaling regulates adult post-hepatectomy regeneration. Thus, Dr. Linda Lorenz and her colleagues at Heinrich-Heine University have defined a signaling pathway in vascular endothelial cells that translates information from blood perfusion and ‘‘mechano-transduction’’ into cellular programs that ultimately control liver growth and maintenance. Fundamentally, their results demonstrate that particular cell types require a given level of mechanical force to maintain tissue homeostasis and to promote tissue growth. One intriguing implication from this paper is that ITGB1dependent signaling, when specifically activated in hepatic vascular endothelial cells, can indirectly translate a mechanical influence to hepatocytes. It will be exciting for future work to characterize the level of mechanical force required to exert such effects in hepatocytes and determine whether such mechanical responses in hepatocytes also somehow contribute to liver growth. More broadly, considering that all cells in an organism are in some sense constantly exposed to various mechanical forces, it seems likely that regulatory roles for mechanical force are all pervasive across not just development but all biological processes.
Developmental Cell 47, December 17, 2018 ª 2018 Elsevier Inc. 673
Developmental Cell
Spotlight Indeed, many organs have unique mechanical properties and are exposed to ever-present mechanical forces. Thus, gaining knowledge about the levels of mechanical force in vivo—in normal physiological and pathological conditions—will be extremely important as developmental biologists strive toward further understanding of the mechanisms underlying development, regeneration, and disease. Such experimental work remains very technically challenging. Molecular force sensors, atomic force microscopy, and many other advanced tools, together with the enormous legacy of available biochemical probes, have facilitated the analysis of the functional contributions of mechanical forces (Nelson, 2017; Sugimura et al., 2016). However, these approaches remain limited in their ability to directly characterize all mechanical contributions given the potentially omnipresent influence of mechanical force. Additional interdisciplinary research that combines modern molecular and cellular biology methods with advanced cell mechanical characterization techniques is greatly needed to deepen our understanding of the regulatory influences of mechanical forces in three-dimensional environments.
Given that mechanical force is, by definition, inherently dynamic, it is very difficult to accurately monitor the immediate cellular behaviors that occur in responses to the dynamic changes of mechanical force, particularly in vivo. In this light, it is not surprising that many mechanical studies have begun to employ mathematical modeling approaches to test hypotheses on the nature of the interaction between complex biological responses and mechanical forces (Irvine and Shraiman, 2017). A new generation of sophisticated live-imaging techniques (Li et al., 2018; Miller et al., 2017)—some of which would be scarcely believable to past generations of scientists like Dr. Julius Wolff—is finally enabling biologists to begin to properly consider questions about the interface of biophysics, cell biology, and developmental biology. Future studies using a blend of approaches from biophysics, mathematical modeling, live imaging, and genetics will certainly lead to discoveries that could not be revealed via genetics approaches alone.
REFERENCES Guillot, C., and Lecuit, T. (2013). Mechanics of epithelial tissue homeostasis and morphogenesis. Science 340, 1185–1189.
674 Developmental Cell 47, December 17, 2018
Irvine, K.D., and Shraiman, B.I. (2017). Mechanical control of growth: ideas, facts and challenges. Development 144, 4238–4248. Li, J., Wang, Z., Chu, Q., Jiang, K., Li, J., and Tang, N. (2018). The strength of mechanical forces determines the differentiation of alveolar epithelial cells. Dev. Cell 44, 297–312.e5, e295. Lorenz, L., Axnick, J., Buschmann, T., Henning, C., Urner, S., Fang, S., Nurmi, H., Eichhorst, N., Holtmeier, R., Bo´dis, K., et al. (2018). Mechanosensing by b1 integrin induces angiocrine signals for liver growth and survival. Nature 562, 128–132. Miller, D.R., Jarrett, J.W., Hassan, A.M., and Dunn, A.K. (2017). Deep tissue imaging with multiphoton fluorescence microscopy. Curr. Opin. Biomed. Eng. 4, 32–39. Nelson, C.M. (2017). From static to animated: measuring mechanical forces in tissues. J. Cell Biol. 216, 29–30. Petridou, N.I., Spiro´, Z., and Heisenberg, C.P. (2017). Multiscale force sensing in development. Nat. Cell Biol. 19, 581–588. , B., Goedecke, A., Breier, G., Planas-Paz, L., Strilic €ssler, R., and Lammert, E. (2012). MechanoinFa duction of lymph vessel expansion. EMBO J. 31, 788–804. Sugimura, K., Lenne, P.F., and Graner, F. (2016). Measuring forces and stresses in situ in living tissues. Development 143, 186–196. Vining, K.H., and Mooney, D.J. (2017). Mechanical forces direct stem cell behaviour in development and regeneration. Nat. Rev. Mol. Cell Biol. 18, 728–742.
Spotlight May the Force Be with You Jiao Li1 and Nan Tang1,* 1National Institute of Biological Sciences, Beijing 102206, China *Correspondence: [email protected] https://doi.org/10.1016/j.devcel.2018.11.041
Elucidating the mechanical force-induced regulatory machineries is extremely important for further understanding of the mechanisms underlying development, regeneration, and disease. Recent work demonstrates that mechanical forces generated by blood flow in liver vessels function to regulate embryonic liver development and may also contribute to regeneration in adult liver. More than a century ago, the German anatomist and surgeon Julius Wolff boldly stated the view that bones can remodel themselves over time so that they become stronger as loading force increases. Consistent with this early idea, it is now known that a variety of mechanical forces, including shear forces caused by fluid viscosity, compression or traction forces exerted by surrounding cells, and even the rigidity of the extracellular matrix, are translated into variable biochemical signals in cells. Specific cellular, biochemical, and mechanical players are required to orchestrate physiology at the subcellular, cellular, tissue, organ, and whole-animal levels during embryonic development, homeostasis, and regeneration (Guillot and Lecuit, 2013; Petridou et al., 2017; Vining and Mooney, 2017). Identifying these players and understanding the complex crosstalk among them is essential for the elucidation of the mechanical force-induced regulatory machineries underlying elegantly sophisticated developmental processes. Reporting recently in Nature, Lorenz et al. (2018) demonstrated that mechanical forces generated by blood flow in liver vessels function to regulate embryonic liver development and, moreover, may also contribute to regeneration in the adult liver. The authors first noticed that murine embryonic livers expand substantially between embryonic days 12.5 (E12.5) and E13.5. Their careful observations also revealed that blood starts to perfuse into the blood vessels of the liver at E11.5 and does so in a highly spatially controlled manner: starting from the periphery and proceeding toward the center of the liver. These observations, viewed alongside their additional finding that proliferating hepatocytes are preferentially distributed at peripheral regions of livers at E11.5,
led the authors to propose that vascular perfusion may induce the proliferation of hepatocytes and thereby subsequently drive liver development and possibly even regeneration. It was previously appreciated that shear forces generated by dynamic blood flows can induce mechanical signals in endothelial cells. The authors therefore investigated whether any signaling events are activated specifically by blood perfusion in liver sinusoidal endothelial cells (LSECs). In light of the previous discovery that VEGFR3 activation in lymphatic endothelial cells requires sheer force-mediated activation of b1 integrin (ITGB1) (PlanasPaz et al., 2012), the authors investigated whether ITGB1 and VEGFR3 are also activated in LSECs in embryonic livers at E11.5 and indeed found that the activated forms of both ITGFB1 and VEGFR3 were conspicuously abundant in the peripheral liver regions that are characterized by both extensive blood perfusion and hyperproliferation of hepatocytes. Next, their loss-of-perfusion and gainof-perfusion experiments showed that the activation of both ITGB1 and VEGFR3 in LSECs was significantly reduced when blood perfusion was blocked by pharmacologically halting the embryo’s heartbeat. Conversely, increased ITGB1 and VEGFR3 activation occurred when blood perfusion was accelerated by increasing the embryo’s heart rate. The genetic evidence showing that impaired liver development occurs in LSEC Itgb1-deficient and Vegfr3-deficient mouse embryos collectively demonstrates that the mechanical force generated by blood perfusion does indeed activate ITGB1/VEGFR3/ HGF signaling in LSECs and consequently promotes embryonic liver development. The authors provide three lines of evidence suggesting that a similar mecha-
nism might occur during adult liver regrowth: (1) both ex vivo and in vitro mechanical stretching treatments activated ITGFB1/VEGF3/HGF signaling in adult hepatic ECs; (2) primary human hepatocytes cultured in medium collected from stretched human LSECs displayed increased proliferation and decreased apoptosis as compared to those in medium from non-stretched human LSECs; (3) in healthy adult humans, liver volume is positively correlated with the blood pressure. These facts collectively suggest that sheer-force-activated endothelial ITGFB1/VEGF3/HGF signaling regulates adult post-hepatectomy regeneration. Thus, Dr. Linda Lorenz and her colleagues at Heinrich-Heine University have defined a signaling pathway in vascular endothelial cells that translates information from blood perfusion and ‘‘mechano-transduction’’ into cellular programs that ultimately control liver growth and maintenance. Fundamentally, their results demonstrate that particular cell types require a given level of mechanical force to maintain tissue homeostasis and to promote tissue growth. One intriguing implication from this paper is that ITGB1dependent signaling, when specifically activated in hepatic vascular endothelial cells, can indirectly translate a mechanical influence to hepatocytes. It will be exciting for future work to characterize the level of mechanical force required to exert such effects in hepatocytes and determine whether such mechanical responses in hepatocytes also somehow contribute to liver growth. More broadly, considering that all cells in an organism are in some sense constantly exposed to various mechanical forces, it seems likely that regulatory roles for mechanical force are all pervasive across not just development but all biological processes.
Developmental Cell 47, December 17, 2018 ª 2018 Elsevier Inc. 673
Developmental Cell
Spotlight Indeed, many organs have unique mechanical properties and are exposed to ever-present mechanical forces. Thus, gaining knowledge about the levels of mechanical force in vivo—in normal physiological and pathological conditions—will be extremely important as developmental biologists strive toward further understanding of the mechanisms underlying development, regeneration, and disease. Such experimental work remains very technically challenging. Molecular force sensors, atomic force microscopy, and many other advanced tools, together with the enormous legacy of available biochemical probes, have facilitated the analysis of the functional contributions of mechanical forces (Nelson, 2017; Sugimura et al., 2016). However, these approaches remain limited in their ability to directly characterize all mechanical contributions given the potentially omnipresent influence of mechanical force. Additional interdisciplinary research that combines modern molecular and cellular biology methods with advanced cell mechanical characterization techniques is greatly needed to deepen our understanding of the regulatory influences of mechanical forces in three-dimensional environments.
Given that mechanical force is, by definition, inherently dynamic, it is very difficult to accurately monitor the immediate cellular behaviors that occur in responses to the dynamic changes of mechanical force, particularly in vivo. In this light, it is not surprising that many mechanical studies have begun to employ mathematical modeling approaches to test hypotheses on the nature of the interaction between complex biological responses and mechanical forces (Irvine and Shraiman, 2017). A new generation of sophisticated live-imaging techniques (Li et al., 2018; Miller et al., 2017)—some of which would be scarcely believable to past generations of scientists like Dr. Julius Wolff—is finally enabling biologists to begin to properly consider questions about the interface of biophysics, cell biology, and developmental biology. Future studies using a blend of approaches from biophysics, mathematical modeling, live imaging, and genetics will certainly lead to discoveries that could not be revealed via genetics approaches alone.
REFERENCES Guillot, C., and Lecuit, T. (2013). Mechanics of epithelial tissue homeostasis and morphogenesis. Science 340, 1185–1189.
674 Developmental Cell 47, December 17, 2018
Irvine, K.D., and Shraiman, B.I. (2017). Mechanical control of growth: ideas, facts and challenges. Development 144, 4238–4248. Li, J., Wang, Z., Chu, Q., Jiang, K., Li, J., and Tang, N. (2018). The strength of mechanical forces determines the differentiation of alveolar epithelial cells. Dev. Cell 44, 297–312.e5, e295. Lorenz, L., Axnick, J., Buschmann, T., Henning, C., Urner, S., Fang, S., Nurmi, H., Eichhorst, N., Holtmeier, R., Bo´dis, K., et al. (2018). Mechanosensing by b1 integrin induces angiocrine signals for liver growth and survival. Nature 562, 128–132. Miller, D.R., Jarrett, J.W., Hassan, A.M., and Dunn, A.K. (2017). Deep tissue imaging with multiphoton fluorescence microscopy. Curr. Opin. Biomed. Eng. 4, 32–39. Nelson, C.M. (2017). From static to animated: measuring mechanical forces in tissues. J. Cell Biol. 216, 29–30. Petridou, N.I., Spiro´, Z., and Heisenberg, C.P. (2017). Multiscale force sensing in development. Nat. Cell Biol. 19, 581–588. , B., Goedecke, A., Breier, G., Planas-Paz, L., Strilic €ssler, R., and Lammert, E. (2012). MechanoinFa duction of lymph vessel expansion. EMBO J. 31, 788–804. Sugimura, K., Lenne, P.F., and Graner, F. (2016). Measuring forces and stresses in situ in living tissues. Development 143, 186–196. Vining, K.H., and Mooney, D.J. (2017). Mechanical forces direct stem cell behaviour in development and regeneration. Nat. Rev. Mol. Cell Biol. 18, 728–742.