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Igoshin OA, Chen J, Xing J, Liu J, Elston TC, Grabe M, Kim KS, Nirody JA, Rangamani P, Sun SX, Wang H, Wolgemuth C. Biophysics at the coffee shop: lessons learned working with George Oster. Mol Biol Cell 2019; 30:1882-1889. [PMID: 31322997 PMCID: PMC6727762 DOI: 10.1091/mbc.e19-02-0107] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
Abstract
Over the past 50 years, the use of mathematical models, derived from physical reasoning, to describe molecular and cellular systems has evolved from an art of the few to a cornerstone of biological inquiry. George Oster stood out as a pioneer of this paradigm shift from descriptive to quantitative biology not only through his numerous research accomplishments, but also through the many students and postdocs he mentored over his long career. Those of us fortunate enough to have worked with George agree that his sharp intellect, physical intuition, and passion for scientific inquiry not only inspired us as scientists but also greatly influenced the way we conduct research. We would like to share a few important lessons we learned from George in honor of his memory and with the hope that they may inspire future generations of scientists.
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Affiliation(s)
- Oleg A Igoshin
- Departments of Bioengineering, Biosciences, and Chemistry and Center for Theoretical Biological Physics, Rice University, Houston, TX 77005
| | - Jing Chen
- Department of Biological Sciences, Virginia Tech, Blacksburg, VA 24061
| | - Jianhua Xing
- Department of Computational and Systems Biology and UPMC-Hillman Cancer Center, School of Medicine, University of Pittsburgh, Pittsburgh, PA 15261
| | - Jian Liu
- Department of Cell Biology and Center for Cell Dynamics, School of Medicine, Johns Hopkins University, Baltimore, MD 21205
| | - Timothy C Elston
- Department of Pharmacology, School of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599
| | - Michael Grabe
- Cardiovascular Research Institute, School of Pharmacy, University of California, San Francisco, San Francisco, CA 94158
| | - Kenneth S Kim
- Lawrence Livermore National Laboratory, Livermore, CA 94550
| | - Jasmine A Nirody
- Center for Studies in Physics and Biology, Rockefeller University, New York, NY 10065
| | - Padmini Rangamani
- Department of Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, CA 92093
| | - Sean X Sun
- Department of Mechanical Engineering, Whiting School of Engineering, Johns Hopkins University, Baltimore, MD 21218
| | - Hongyun Wang
- Department of Applied Mathematics and Statistics, University of California, Santa Cruz, Santa Cruz, CA 95064
| | - Charles Wolgemuth
- Department of Physics and Department of Molecular and Cellular Biology, University of Arizona, Tucson, AZ 85721
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Abstract
Statistical analysis is applied to a set of electron micrographic images (Kocsis, E., B. L. Trus, C. J. Steer, M. E. Bisher, and A. C. Steven. 1991. J. Struct. Biol. 107:6-14), from which quantitative measures are obtained to support the notion that the three arms of a triskelion have statistically identical properties and exhibit independent structural fluctuations. Additionally, a study of local contour fluctuations, which indicates that the elastic properties of a triskelion arm are approximately constant over the entire arm length, is used along with a small deformation statistical mechanics theory to derive an effective, average flexural rigidity for the arms. This result is used to estimate the bending energy necessary to deform a clathrin patch, and comparison is made with the deformation energy of an equivalent area of non-clathrin-coated membrane. We estimate that the rigidity of the clathrin lattice is at least comparable to that of a typical membrane. Hence, the natural curvature of a clathrin cage can stabilize, and perhaps propel, the formation of intracellular coated vesicles.
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Affiliation(s)
- A J Jin
- Laboratory of Physical Biology, National Institute of Arthritis and Musculoskeletal and Skin Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA
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Mui BL, Döbereiner HG, Madden TD, Cullis PR. Influence of transbilayer area asymmetry on the morphology of large unilamellar vesicles. Biophys J 1995; 69:930-41. [PMID: 8519993 PMCID: PMC1236322 DOI: 10.1016/s0006-3495(95)79967-2] [Citation(s) in RCA: 110] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023] Open
Abstract
The morphological consequences of differences in the monolayer surface areas of large unilamellar vesicles (LUVs) have been examined employing cryoelectron microscopy techniques. Surface area was varied by inducing net transbilayer transport of dioleoylphosphatidylglycerol (DOPG) in dioleoylphosphatidylcholine (DOPC):DOPG (9:1, mol:mol) LUVs in response to transmembrane pH gradients. It is shown that when DOPG is transported from the inner to the outer monolayer, initially invaginated LUVs are transformed to long narrow tubular structures, or spherical structures with one or more protrusions. Tubular structures are also seen in response to outward DOPG transport in DOPC:DOPG:Chol (6:1:3, mol:mol:mol) LUV systems, and when lyso-PC is allowed to partition into the exterior monolayer of DOPC:DOPG (9:1, mol:mol) LUVs in the absence of DOPG transport. Conversely, when the inner monolayer area is expanded by the transport of DOPG from the outer monolayer to the inner monolayer of non-invaginated LUVs, a reversion to invaginated structures is observed. The morphological changes are well described by an elastic bending theory of the bilayer. Identification of the difference in relaxed monolayer areas and of the volume-to-area ratio of the LUVs as the shape-determining factors allows a quantitative classification of the observed morphologies. The morphology seen in LUVs supports the possibility that factors leading to differences in monolayer surface areas could play important roles in intracellular membrane transport processes.
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Affiliation(s)
- B L Mui
- Department of Biochemistry, University of British Columbia, Vancouver, Canada
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Thomas JL, Barton SW, Tirrell DA. Membrane solubilization by a hydrophobic polyelectrolyte: surface activity and membrane binding. Biophys J 1994; 67:1101-6. [PMID: 7811920 PMCID: PMC1225462 DOI: 10.1016/s0006-3495(94)80575-2] [Citation(s) in RCA: 66] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
We have previously observed that the hydrophobic polyelectrolyte poly(2-ethylacrylic acid) solubilizes lipid membranes in a pH-dependent manner, and we have exploited this phenomenon to prepare lipid vesicles that release their contents in response to pH, light, or glucose (Thomas, J. L., and D. A. Tirrell. Acc. Chem. Res. 25:336-342, 1992). The physical basis for the interaction between poly(2-ethylacrylic acid) and lipid membranes has been explored using surface tensiometry and fluorimetry. Varying the polymer concentration results in changes in surface activity and membrane binding that correlate with shifts in the critical pH for membrane solubilization. Furthermore, the binding affinity is reduced as the amount of bound polymer increases. These results are consistent with a hydrophobically driven micellization process, similar to those observed with apolipoproteins, melittin, and other amphiphilic alpha-helix-based polypeptides. The absence of specific secondary structure in the synthetic polymer suggests that amphiphilicity, rather than structure, is the most important factor in membrane micellization by macromolecules.
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Affiliation(s)
- J L Thomas
- Department of Polymer Science and Engineering, University of Massachusetts, Amherst 01003
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Sheffield JB, Krasnopolsky V, Dehlinger E. Inhibition of retinal growth cone activity by specific metalloproteinase inhibitors in vitro. Dev Dyn 1994; 200:79-88. [PMID: 8081016 DOI: 10.1002/aja.1002000108] [Citation(s) in RCA: 27] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
The developing neural retina expresses a set of extracellular proteases including plasminogen activator and gelatinases. Since neurites of retina cells cultured on fluorescent gelatin digest the substrate in their paths, we have suggested that the proteases are used by the tips of growing fibers to allow them to migrate within the mass of the tissue in vivo. In order to obtain further information about relationships between extracellular proteases and fiber growth, we have examined the effects of the specific inhibitors HS-LFA (HS-Leu-Phenylala-Ala, enantiomeric forms 1 and 2), bathophenanthroline sulfonate (BPS), phenylmethyl sulfonyl fluoride (PMSF), and relevant controls on the activity of retinal growth cones in vitro, monitored by time lapse video microscopy. Of the inhibitors tested, only the two enantiomeric forms of HS-LFA caused a reproducible cessation of both spike extension and filopodial processes at the growth cone ruffling, while control media had no effect. In some cases, the growth cone swelled and exhibited small protrusions. The behavior of growth cones was in sharp distinction to that of the cytoplasm of neural cells, and membrane ruffling of flat cells, which continued in activity throughout. Growth cone activity returned after several hours in the presence of the agent. BPS was toxic at concentrations above 2.5 mM. Below that, it had no effect. L-cysteine, PMSF, and control media had no effect. The relevance of these results to the possible role of proteases in fiber outgrowth from retinal cells is discussed.
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Affiliation(s)
- J B Sheffield
- Department of Biology, Temple University, Philadelphia, Pennsylvania 19122
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Abstract
We report on budding and fission of protein-free vesicles swollen from a natural lipid mixture of bovine brain sphingomyelins. Budding was induced by increasing the area-to-volume ratio through heating. Morphological changes were monitored by phase contrast microscopy and correlated with the thermal behavior of the bilayer by differential scanning calorimetry. Freeze fracture electron microscopy revealed that budding and fission are not restricted to giant vesicles but also occur on length scales relevant for cellular processes. We also observed osmotically induced budding and fission in mixtures of dimyristoyl phosphatidylcholine with cholesterol. We find that these shape transitions are driven by liquid/gel domain formation and/or coupling of the spontaneous curvature of the membrane to the local lipid composition. Our results provide evidence that coat proteins are not necessary for budding and fission of vesicles. The physics of the lipid bilayer is rich enough to explain the observed behavior.
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Affiliation(s)
- H G Döbereiner
- Department of Physics, Simon Fraser University, Burnaby, British Columbia, Canada
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Abstract
How does a virus bud from the plasma membrane of its host? Here we investigate several possible rate-limiting processes, including thermal fluctuations of the plasma membrane, hydrodynamic interactions, and diffusion of the glycoprotein spikes. We find that for bending moduli greater than 3 x 10(-13) ergs, membrane thermal fluctuations are insufficient to wrap the viral capsid, and the mechanical force driving the budding process must arise from some other process. If budding is limited by the rate at which glycoprotein spikes can diffuse to the budding site, we compute that the budding time is 10-20 min, in accord with the experimentally determined upper limit of 20 min. In light of this, we suggest some alternative mechanisms for budding and provide a rationale for the observation that budding frequently occurs in regions of high membrane curvature.
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Affiliation(s)
- D M Lerner
- Department of Molecular and Cell Biology, University of California, Berkeley 94720
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Abstract
In developmental biology, binary cell-cell interactions often determine the fate of one or both cell partners. The two cells must adhere to one another to allow chemical signals to be transmitted in one or both directions across the regions of cell-cell contact. The molecular mechanisms of cell-cell adhesion and intercellular communication, even if they are mediated by different cell surface components, may be functionally integrated in several different ways. Studies of helper T cells with antigen-presenting B cells in culture have illuminated such binary interactions. The possible application of similar mechanisms to other binary developmental systems is briefly explored.
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Affiliation(s)
- S J Singer
- Department of Biology, University of California, San Diego, La Jolla 92093
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Use of a structured kinetic model of antibody synthesis and secretion for optimization of antibody production systems: I. Steady-state analysis. Biotechnol Bioeng 1992; 39:251-61. [DOI: 10.1002/bit.260390302] [Citation(s) in RCA: 42] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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Flickinger MC, Goebel NK, Bibila T, Boyce-Jacino S. Evidence for posttranscriptional stimulation of monoclonal antibody secretion by l-glutamine during slow hybridoma growth. J Biotechnol 1992; 22:201-26. [PMID: 1367980 DOI: 10.1016/0168-1656(92)90142-v] [Citation(s) in RCA: 43] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Abstract
The addition of 5-40 mM L-glutamine to batch cultures of a murine hybridoma following the cessation of rapid growth significantly stimulated monoclonal antibody (mAb) synthesis and secretion per cell. Stimulation of mAb secretion following the cessation of rapid growth was also observed in response to addition of mitochondrial intermediates of glutamate oxidation and was not found to be the result of release of transiently stored mAb. Less than 1% of the secreted mAb was detected by ELISA in isolated hybridoma lysosomes. This stimulation was posttranscriptional and not the result of enhancement of levels of mAb mRNAs or stabilization of heavy (H) or light (L) chain encoding message. Sub-inhibitory levels of lysosomotrophic weak bases stimulated release of lysosomal contents but did not result in release of intact or partially degraded mAb. Inhibition of aspartic proteinase activity secreted by the hybridoma did not enhance mAb secretion even though a high level of mAb degrading proteinase activity was continuously secreted during both rapid and slow growth. These responses indicate that during slow growth, the addition of L-glutamine increases the availability of cellular ATP generated by mitochondrial respiration which stimulates some posttranscriptional step in the pathway of mAb secretion such as the rate of H or L chain translation, chain assembly, interorganelle transport or vesicular transport from the Golgi to the cell membrane.
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Affiliation(s)
- M C Flickinger
- Department of Biochemistry, University of Minnesota, St. Paul 55108
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Abstract
Transfer of proteins and lipids between the various membrane-bound subcellular compartments of the eukaryotic cell is mediated by transport vesicles. The development of cell-free assays has allowed rapid progress towards a molecular description of the formation, or budding, of these vesicles. This article reviews and integrates data obtained from various yeast and mammalian systems on molecules involved in the budding reaction.
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Affiliation(s)
- P Melançon
- Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA
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