Samuel Kahn

University of Pennsylvania, Philadelphia, Pennsylvania, United States

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Publications (4)12.99 Total impact

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    ABSTRACT: Strong adhesion of highly active cells often nucleates focal adhesions, synapses, and related structures. Red cells lack such complex adhesion systems and are also nonmotile, but they are shown here to dynamically evolve complex spatial patterns beyond an electrostatic threshold for strong adhesion. Spreading of the cell onto a dense, homogeneous poly-L-lysine surface appears complete in <1 s with occasional blisters that form and dissipate on a similar timescale; distinct rippled or stippled patterns in fluorescently labeled membrane components emerge later, however, on timescales more typical of long-range lipid diffusion (approximately minutes). Within the contact zone, the anionic fluorescent lipid fluorescein phosphoethanolamine is seen to rearrange, forming worm-like rippled or stippled domains of <500 nm that prove independent of whether the cell is intact and sustaining a tension or ruptured. Lipid patterns are accompanied by visible perturbations in Band 3 distribution and weaker perturbations in membrane skeleton actin. Pressing down on the membrane quenches the lipid patterns, revealing a clear topographical basis for pattern formation. Counterion screening and membrane fluctuations likely contribute, but the results primarily highlight the fact that even in adhesion of a passive red cell, regions of strong contact slowly evolve to become interspersed with regions where the membrane is more distant from the surface.
    Biophysical Journal 12/2004; 87(5):3547-60. · 3.83 Impact Factor
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    ABSTRACT: Mechanical properties of the nuclear envelope have implications for cell and nuclear architecture as well as gene regulation. Using isolated Xenopus oocyte nuclei, we have established swelling conditions that separate the intact nuclear envelope (membranes, pore complexes and underlying lamin filament network) from nucleoplasm and the majority of chromatin. Swelling proves reversible with addition of high molecular mass dextrans. Micropipette aspiration of swollen and unswollen nuclear envelopes is also reversible and yields a network elastic modulus, unaffected by nucleoplasm, that averages 25 mN/m. Compared to plasma membranes of cells, the nuclear envelope is much stiffer and more resilient. Our results suggest that the nuclear lamina forms a compressed network shell of interconnected rods that is extensible but limited in compressibility from the native state, thus acting as a 'molecular shock absorber'. In light of the conservation of B-type lamins in metazoan evolution, the mechanical properties determined in this investigation suggest physical mechanisms by which mutated lamins can either destabilize nuclear architecture or influence nuclear responses to mechanical signals in Emery-Dreifuss muscular dystrophy, cardiomyopathy, progeria syndromes (premature 'aging') and other laminopathies.
    Journal of Cell Science 10/2004; 117(Pt 20):4779-86. · 5.33 Impact Factor
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    ABSTRACT: Membrane tension underlies a range of cell physiological processes. Strong adhesion of the simple red cell is used as a simple model of a spread cell with a finite membrane tension-a state which proves useful for studies of both membrane rupture kinetics and atomic force microscopy (AFM) probing of native structure. In agreement with theories of strong adhesion, the cell takes the form of a spherical cap on a substrate densely coated with poly-L-lysine. The spreading-induced tension, sigma, in the membrane is approximately 1 mN/m, which leads to rupture over many minutes; and sigma is estimated from comparable rupture times in separate micropipette aspiration experiments. Under the sharpened tip of an AFM probe, nano-Newton impingement forces (10-30 nN) are needed to penetrate the tensed erythrocyte membrane, and these forces increase exponentially with tip velocity ( approximately nm/ms). We use the results to clarify how tapping-mode AFM imaging works at high enough tip velocities to avoid rupturing the membrane while progressively compressing it to a approximately 20-nm steric core of lipid and protein. We also demonstrate novel, reproducible AFM imaging of tension-supported membranes in physiological buffer, and we describe a stable, distended network consistent with the spectrin cytoskeleton. Additionally, slow retraction of the AFM tip from the tensed membrane yields tether-extended, multipeak sawtooth patterns of average force approximately 200 pN. In sum we show how adhesive tensioning of the red cell can be used to gain novel insights into native membrane dynamics and structure.
    Biophysical Journal 11/2003; 85(4):2746-59. · 3.83 Impact Factor
  • K.N. Dahl, S.M. Kahn, D.E. Discher
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    ABSTRACT: Deformation of the nucleus has been suggested to be a significant step in mechanotransduction. Additionally, many established and emerging technologies exploit the ability to isolate, transfer and physically manipulate nuclei. This study focuses on providing a fundamental understanding of the mechanical properties of isolated nuclei and nuclear components. Micropipette aspiration allows for relations of stresses to strains and was used to determine mechanical properties of nuclei isolated from Xenopus laevis oocytes. Preliminary results suggest that nuclear membranes can exhibit extensional strains of up to 100%. The elastic modulus of these nuclei appear to be an order of magnitude higher than previously studied cellular membrane systems, such as erythrocytes, suggesting a rigid underlying lamin network. Micropipette aspiration combined with fluorescence labeling allows for the visualization of individual components during deformation. Fluorescent labeling of DNA within isolated nuclei suggests that the cohesive forces are stronger than the forces holding the chromatin to the nuclear membrane. The deformation of the chromatin within the nucleus does not have any significant effect on the mechanical properties of the nuclear membrane. Fluorescent antibody labeling of nuclear pores of mammalian nuclei suggests that there is significant attachment of the pores to the underlying lamina network.
    Engineering in Medicine and Biology, 2002. 24th Annual Conference and the Annual Fall Meeting of the Biomedical Engineering Society EMBS/BMES Conference, 2002. Proceedings of the Second Joint; 02/2002