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Zebrafish Keep Their Cool
Please cite this article in press as: Honerkamp-Smith, Zebrafish Keep Their Cool, Biophysical Journal (2017), http://dx.doi.org/10.1016/j.bpj.2017.05.003
New and Notable
Zebrafish Keep Their Cool Aurelia R. Honerkamp-Smith1,* 1
Department of Physics, Lehigh University, Bethlehem, Pennsylvania
Cell plasma membranes contain a complex assortment of lipids. This mixture is metabolically tuned to respond to changes in environmental conditions, but the specific biological function of lipid adaptation is not understood. In their newest article, Burns et al. (1) address this longstanding question with experimental evidence that cells alter their lipid composition to maintain a constant difference between the growth temperature and the temperature at which their membranes separate into coexisting liquid phases. This intriguing result has the potential to significantly advance the field of lipid biology. The function, dynamic properties and spatial properties, and the very existence of lipid composition heterogeneity in biological lipid membranes, are topics that have inspired extensive discussion. After detergent-resistant fractions derived from biological membranes were first linked with coexisting liquid phases in model membranes (2,3), many researchers have sought to characterize them in biological and model lipid bilayers (4–6). Multiple links between lipid heterogeneity and biological function have been proposed, but the search for an overarching description continues. The centrality of lipid membranes to multiple biological processes further
Submitted March 31, 2017, and accepted for publication May 4, 2017. *Correspondence: [email protected] Editor: Claudia Steinem.
muddies the waters. One prediction that follows from functional heterogeneity is that cells will seek to maintain a lipid composition that retains function in changing environments. Multiple organisms modify their overall lipid composition when temperature changes (7), as well as in response to other environmental changes (8). Membrane viscosity, bending stiffness, thickness, and lipid diffusion constants all vary with temperature, and each property can in turn alter protein function and signaling. A further challenge arises from the intrinsic variability in biological membranes, between different samples and even between individual cells. Currently, there is no compelling single theory to explain lipid composition shifts, in response to temperature or to other environmental cues. Previous work demonstrated that giant plasma membrane vesicles (GPMVs) derived from the membrane of living cells undergo a critical liquid-demixing phase transition when cooled below the cell’s growth temperature (9). Additional experiments have supported the feasibility of submicron fluctuations as functional biological phenomena and described their consequences for protein oligomerization and regulation (10–14). Proximity to the critical temperature provides an attractive rationale, based on fundamental physics, for lipid adaptation. Critical transition temperatures are more sensitive to composition changes than the chain melting transition or viscosities, and can vary counterintuitively
in complicated multicomponent mixtures. However, it is a formidable problem to determine whether this particular physical property is germane to the function of living organisms (Fig. 1). The experiments described in this issue of the Biophysical Journal address this difficulty in an admirably direct way. Burns et al. (1) demonstrate that a zebrafish cell line grown at different temperatures yields GPMVs that phase separate at different temperatures. In fact, the change in the transition temperature is the same as the change in growth temperature, so that cells maintain a constant offset between their growth temperature and the transition temperature of GPMVs derived from the cells. Although intuitive to understand, these results were not easy to obtain. GPMVs isolated from living cells display a large amount of intrinsic variation, yet the researchers were able to identify robust trends. To refine their results, the authors use lipidomics to identify specific lipid types that increase and decrease with growth temperature, with surprising results. As expected, changes in transition temperature are driven by changes in lipid composition. But in some cases, the concentration trend is opposite to that predicted from the well-known phase diagrams of threecomponent lipid mixtures. This experiment serves as a reminder that the thermodynamic behavior of the complex, physiologically relevant compositions found in GPMVs cannot
http://dx.doi.org/10.1016/j.bpj.2017.05.003 Ó 2017 Biophysical Society.
Biophysical Journal 113, 1–2, September 19, 2017 1
Please cite this article in press as: Honerkamp-Smith, Zebrafish Keep Their Cool, Biophysical Journal (2017), http://dx.doi.org/10.1016/j.bpj.2017.05.003
Honerkamp-Smith September 19, 2017. http://dx.doi.org/10. 1016/j.bpj.2017.04.052. 2. Dietrich, C., L. A. Bagatolli, ., E. Gratton. 2001. Lipid rafts reconstituted in model membranes. Biophys. J. 80:1417–1428. 3. Simons, K., and W. L. Vaz. 2004. Model systems, lipid rafts, and cell membranes. Annu. Rev. Biophys. Biomol. Struct. 33:269–295. 4. Samsonov, A. V., I. Mihalyov, and F. S. Cohen. 2001. Characterization of cholesterol-sphingomyelin domains and their dynamics in bilayer membranes. Biophys. J. 81:1486–1500. 5. Veatch, S. L., K. Gawrisch, and S. L. Keller. 2006. Closed-loop miscibility gap and quantitative tie-lines in ternary membranes containing diphytanoyl PC. Biophys. J. 90:4428–4436. 6. Bacia, K., D. Scherfeld, ., P. Schwille. 2004. Fluorescence correlation spectroscopy relates rafts in model and native membranes. Biophys. J. 87:1034–1043.
FIGURE 1 Fractal patterns appear on homogeneous giant unilamellar vesicles (inset images, z40 mm wide) when their temperature approaches the critical miscibility temperature (Tm). Below Tm, round domains indicate liquid-liquid coexistence. Similarly, vesicles derived from the plasma membrane of living cells (zebrafish cell line ZF4) exhibit critical phase transitions at fixed temperatures relative to their growth temperatures, as described in this issue by Burns et al. (1). Here the phase separation patterns are repeated on the zebrafish sketches for artistic purposes; in reality, zebrafish stripes arise from cell-cell communication (15), not lipid phase coexistence. This image was created by Aurelia and Savanna Honerkamp-Smith. To see this figure in color, go online.
always be predicted from that observed in simpler ones. It also illustrates the utility of filling out the big picture described by membrane thermodynamic properties with the molecular detail provided by lipidomics. Deciphering the functional significance of down- versus upregulation of particular lipid species during temperature adaptation will be an ongoing challenge for lipid biochemists. Conserved adaptation of lipids demonstrates that they are a crucial, regulated cell component rather than a passive substrate for membrane proteins. This experiment is a significant addition to the growing body of experiments and theory demonstrating molecular mechanisms by which critical fluctuations can regulate biological function. The field of membrane biophysics is poised to deliver broad insights in cell and lipid biology, and with
each advance, further possibilities come into view. For instance, understanding the essential features of short-term lipid composition changes, like the one described here, can also shed light on longer-term or interspecies lipid variation and illuminate the role of lipid composition in evolutionary biology. This type of insightful, interdisciplinary work is currently driving the field of lipid biophysics. As more evidence for the biological significance of lipid demixing emerges, the underlying logic of this complicated web of physical and chemical interactions will become clear. REFERENCES 1. Burns, M., K. Wisser, ., S. L. Veatch. 2017. Miscibility transition temperature scales with growth temperature in a zebrafish cell line. Biophys. J. 113 Published online:
2 Biophysical Journal 113, 1–2, September 19, 2017
7. Hazel, J. R. 1995. Thermal adaptation in biological membranes: is homeoviscous adaptation the explanation? Annu. Rev. Physiol. 57:19–42. 8. Gray, E. M., G. Dı´az-Va´zquez, and S. L. Veatch. 2015. Growth conditions and cell cycle phase modulate phase transition temperatures in RBL-2H3 derived plasma membrane vesicles. PLoS One. 10:e0137741. 9. Veatch, S. L., P. Cicuta, ., B. Baird. 2008. Critical fluctuations in plasma membrane vesicles. ACS Chem. Biol. 3:287–293. 10. Machta, B. B., S. Papanikolaou, ., S. L. Veatch. 2011. Minimal model of plasma membrane heterogeneity requires coupling cortical actin to criticality. Biophys. J. 100:1668–1677. 11. Zhao, J., J. Wu, and S. L. Veatch. 2013. Adhesion stabilizes robust lipid heterogeneity in supercritical membranes at physiological temperature. Biophys. J. 104:825–834. 12. Gray, E., J. Karslake, ., S. L. Veatch. 2013. Liquid general anesthetics lower critical temperatures in plasma membrane vesicles. Biophys. J. 105:2751–2759. 13. Machta, B. B., E. Gray, ., S. L. Veatch. 2016. Conditions that stabilize membrane domains also antagonize n-alcohol anesthesia. Biophys. J. 111:537–545. 14. Stone, M. B., S. A. Shelby, ., S. L. Veatch. 2017. Protein sorting by lipid phase-like domains supports emergent signaling function in B lymphocyte plasma membranes. eLife. 6:6. 15. Eom, D. S., E. J. Bain, ., D. M. Parichy. 2015. Long-distance communication by specialized cellular projections during pigment pattern development and evolution. eLife. 4:e12401.
New and Notable
Zebrafish Keep Their Cool Aurelia R. Honerkamp-Smith1,* 1
Department of Physics, Lehigh University, Bethlehem, Pennsylvania
Cell plasma membranes contain a complex assortment of lipids. This mixture is metabolically tuned to respond to changes in environmental conditions, but the specific biological function of lipid adaptation is not understood. In their newest article, Burns et al. (1) address this longstanding question with experimental evidence that cells alter their lipid composition to maintain a constant difference between the growth temperature and the temperature at which their membranes separate into coexisting liquid phases. This intriguing result has the potential to significantly advance the field of lipid biology. The function, dynamic properties and spatial properties, and the very existence of lipid composition heterogeneity in biological lipid membranes, are topics that have inspired extensive discussion. After detergent-resistant fractions derived from biological membranes were first linked with coexisting liquid phases in model membranes (2,3), many researchers have sought to characterize them in biological and model lipid bilayers (4–6). Multiple links between lipid heterogeneity and biological function have been proposed, but the search for an overarching description continues. The centrality of lipid membranes to multiple biological processes further
Submitted March 31, 2017, and accepted for publication May 4, 2017. *Correspondence: [email protected] Editor: Claudia Steinem.
muddies the waters. One prediction that follows from functional heterogeneity is that cells will seek to maintain a lipid composition that retains function in changing environments. Multiple organisms modify their overall lipid composition when temperature changes (7), as well as in response to other environmental changes (8). Membrane viscosity, bending stiffness, thickness, and lipid diffusion constants all vary with temperature, and each property can in turn alter protein function and signaling. A further challenge arises from the intrinsic variability in biological membranes, between different samples and even between individual cells. Currently, there is no compelling single theory to explain lipid composition shifts, in response to temperature or to other environmental cues. Previous work demonstrated that giant plasma membrane vesicles (GPMVs) derived from the membrane of living cells undergo a critical liquid-demixing phase transition when cooled below the cell’s growth temperature (9). Additional experiments have supported the feasibility of submicron fluctuations as functional biological phenomena and described their consequences for protein oligomerization and regulation (10–14). Proximity to the critical temperature provides an attractive rationale, based on fundamental physics, for lipid adaptation. Critical transition temperatures are more sensitive to composition changes than the chain melting transition or viscosities, and can vary counterintuitively
in complicated multicomponent mixtures. However, it is a formidable problem to determine whether this particular physical property is germane to the function of living organisms (Fig. 1). The experiments described in this issue of the Biophysical Journal address this difficulty in an admirably direct way. Burns et al. (1) demonstrate that a zebrafish cell line grown at different temperatures yields GPMVs that phase separate at different temperatures. In fact, the change in the transition temperature is the same as the change in growth temperature, so that cells maintain a constant offset between their growth temperature and the transition temperature of GPMVs derived from the cells. Although intuitive to understand, these results were not easy to obtain. GPMVs isolated from living cells display a large amount of intrinsic variation, yet the researchers were able to identify robust trends. To refine their results, the authors use lipidomics to identify specific lipid types that increase and decrease with growth temperature, with surprising results. As expected, changes in transition temperature are driven by changes in lipid composition. But in some cases, the concentration trend is opposite to that predicted from the well-known phase diagrams of threecomponent lipid mixtures. This experiment serves as a reminder that the thermodynamic behavior of the complex, physiologically relevant compositions found in GPMVs cannot
http://dx.doi.org/10.1016/j.bpj.2017.05.003 Ó 2017 Biophysical Society.
Biophysical Journal 113, 1–2, September 19, 2017 1
Please cite this article in press as: Honerkamp-Smith, Zebrafish Keep Their Cool, Biophysical Journal (2017), http://dx.doi.org/10.1016/j.bpj.2017.05.003
Honerkamp-Smith September 19, 2017. http://dx.doi.org/10. 1016/j.bpj.2017.04.052. 2. Dietrich, C., L. A. Bagatolli, ., E. Gratton. 2001. Lipid rafts reconstituted in model membranes. Biophys. J. 80:1417–1428. 3. Simons, K., and W. L. Vaz. 2004. Model systems, lipid rafts, and cell membranes. Annu. Rev. Biophys. Biomol. Struct. 33:269–295. 4. Samsonov, A. V., I. Mihalyov, and F. S. Cohen. 2001. Characterization of cholesterol-sphingomyelin domains and their dynamics in bilayer membranes. Biophys. J. 81:1486–1500. 5. Veatch, S. L., K. Gawrisch, and S. L. Keller. 2006. Closed-loop miscibility gap and quantitative tie-lines in ternary membranes containing diphytanoyl PC. Biophys. J. 90:4428–4436. 6. Bacia, K., D. Scherfeld, ., P. Schwille. 2004. Fluorescence correlation spectroscopy relates rafts in model and native membranes. Biophys. J. 87:1034–1043.
FIGURE 1 Fractal patterns appear on homogeneous giant unilamellar vesicles (inset images, z40 mm wide) when their temperature approaches the critical miscibility temperature (Tm). Below Tm, round domains indicate liquid-liquid coexistence. Similarly, vesicles derived from the plasma membrane of living cells (zebrafish cell line ZF4) exhibit critical phase transitions at fixed temperatures relative to their growth temperatures, as described in this issue by Burns et al. (1). Here the phase separation patterns are repeated on the zebrafish sketches for artistic purposes; in reality, zebrafish stripes arise from cell-cell communication (15), not lipid phase coexistence. This image was created by Aurelia and Savanna Honerkamp-Smith. To see this figure in color, go online.
always be predicted from that observed in simpler ones. It also illustrates the utility of filling out the big picture described by membrane thermodynamic properties with the molecular detail provided by lipidomics. Deciphering the functional significance of down- versus upregulation of particular lipid species during temperature adaptation will be an ongoing challenge for lipid biochemists. Conserved adaptation of lipids demonstrates that they are a crucial, regulated cell component rather than a passive substrate for membrane proteins. This experiment is a significant addition to the growing body of experiments and theory demonstrating molecular mechanisms by which critical fluctuations can regulate biological function. The field of membrane biophysics is poised to deliver broad insights in cell and lipid biology, and with
each advance, further possibilities come into view. For instance, understanding the essential features of short-term lipid composition changes, like the one described here, can also shed light on longer-term or interspecies lipid variation and illuminate the role of lipid composition in evolutionary biology. This type of insightful, interdisciplinary work is currently driving the field of lipid biophysics. As more evidence for the biological significance of lipid demixing emerges, the underlying logic of this complicated web of physical and chemical interactions will become clear. REFERENCES 1. Burns, M., K. Wisser, ., S. L. Veatch. 2017. Miscibility transition temperature scales with growth temperature in a zebrafish cell line. Biophys. J. 113 Published online:
2 Biophysical Journal 113, 1–2, September 19, 2017
7. Hazel, J. R. 1995. Thermal adaptation in biological membranes: is homeoviscous adaptation the explanation? Annu. Rev. Physiol. 57:19–42. 8. Gray, E. M., G. Dı´az-Va´zquez, and S. L. Veatch. 2015. Growth conditions and cell cycle phase modulate phase transition temperatures in RBL-2H3 derived plasma membrane vesicles. PLoS One. 10:e0137741. 9. Veatch, S. L., P. Cicuta, ., B. Baird. 2008. Critical fluctuations in plasma membrane vesicles. ACS Chem. Biol. 3:287–293. 10. Machta, B. B., S. Papanikolaou, ., S. L. Veatch. 2011. Minimal model of plasma membrane heterogeneity requires coupling cortical actin to criticality. Biophys. J. 100:1668–1677. 11. Zhao, J., J. Wu, and S. L. Veatch. 2013. Adhesion stabilizes robust lipid heterogeneity in supercritical membranes at physiological temperature. Biophys. J. 104:825–834. 12. Gray, E., J. Karslake, ., S. L. Veatch. 2013. Liquid general anesthetics lower critical temperatures in plasma membrane vesicles. Biophys. J. 105:2751–2759. 13. Machta, B. B., E. Gray, ., S. L. Veatch. 2016. Conditions that stabilize membrane domains also antagonize n-alcohol anesthesia. Biophys. J. 111:537–545. 14. Stone, M. B., S. A. Shelby, ., S. L. Veatch. 2017. Protein sorting by lipid phase-like domains supports emergent signaling function in B lymphocyte plasma membranes. eLife. 6:6. 15. Eom, D. S., E. J. Bain, ., D. M. Parichy. 2015. Long-distance communication by specialized cellular projections during pigment pattern development and evolution. eLife. 4:e12401.