Cells, Gels and the Engines of Life

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Focus
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How do cells work?
from the ground up
(water and the cell)
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cell biological concepts today (Alberts et al.
text)

mechanisms based on channels, pumps
channels, pumps for exchange
continuous barrier reqd
dissolved solutes
aqueous soln
cytoplasm
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cell biological concepts today (Alberts et al.
text)

mechanisms based on channels, pumps
channels, pumps for exchange
continuous barrier reqd
dissolved solutes
aqueous soln ?
cytoplasm
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Kellermayer et al., 1986 Cameron et al., 1996 etc.
Conclusion K not freely diffusible
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cell biological concepts today (Alberts et al.
text)

mechanisms based on channels, pumps
channels, pumps for exchange
continuous barrier reqd
dissolved solutes ?
aqueous soln ?
cytoplasm
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cell biological concepts today (Alberts et al.
text)

mechanisms based on channels, pumps
channels, pumps for exchange
continuous barrier reqd ?
dissolved solutes ?
aqueous soln ?
cytoplasm
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Pumps require energy

• Na-pump consumes 30 - 35 of cells energy supply
• gt100 additional cell-membrane pumps
• mitochondrial membrane pumps
• endoplasmic reticular membrane pumps
• etc.
• Is there enough energy?

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Sachs and Qin, 1993
Lev et al., 1993
Woodbury, 1989
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Largest channels and small solutes
membrane
membrane
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cell biological concepts today (Alberts et al.
text)

mechanisms based on channels, pumps
channels, pumps for exchange ?
continuous barrier reqd ?
dissolved solutes ?
aqueous soln ?
cytoplasm
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Begin at bottom gel

• Most abundant component of gel -

Water
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Water is a dipole
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Bonding effect
glue-like behavior
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Is water in the cell structured?
Requirements

• charged surfaces
• surfaces closely packed

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Average surface-to-surface distance lt 5 nm
Literature estimates 1.5 - 2.5 nm
Therefore, conditions ideal for structuring
(Goodsell)
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Evidence that cytoplasmic water is structured

• Nuclear magnetic resonance
• Ultrahigh frequency dielectric dispersion
• Quasi-electric neutron scattering

Details in recent review articles (e.g., Mentre,
1995 Vogler, 1998)
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How does this impact function?
Structured water differs from bulk water
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Static properties of cell

• Ion distribution (in vs. out)
• partitioning insidestructured vs. outside bulk
• association with cell proteinsin
• Cell potential

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Partitioning between structured and bulk water
Larger solutes dig hole with more difficulty than
smaller ones
Energy term
Larger solutes more constrained than smaller ones
Entropy term
Conclusion lgr. solutes partition more
profoundly toward bulk
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Testing size-based solute exclusion
muscle cells
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Ions
ion

hydration shell
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High field strength
Low field strength
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Lower solubility in structured water
Higher solubility in structured water
Lower affinity for charged surfaces
Higher affinity for charged surfaces
Less in cell
More in cell
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Testing this prediction in the cell
radioactively labeled ions
t 0
vaseline coat
t several days
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cut
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Conclusion origin of ion gradients
K 150 Na 10
K 5 Na 150

• Na solute exclusion
• K affinity to protein charges
• similar behavior in gels

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Origin of cell potential?
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How much net charge in cell?
Protein

• Negative charge 1.6 mol/kg
• Positive charge 1.01 mol/kg
• Net protein charge 0.6 mol/kg (-) (Wiggins,
  1990)
• Potassium ion 0.5 mol/kg ()
• Chloride ion 0.2 mol/kg (-)
• Net ion charge 0.3 mol/kg ()
• NET CYTOPLASMIC CHARGE 0.3 mol/kg (-)

Ions
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potentials in gels
(Courtesy, R. Gülch)
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Summary of static features
Pumps not required
Channels not required
Energy not required
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Structured water beyond gel surface?
microspheres
(solutes)
PVA gel
100 µm
Zheng and Pollack, PRE, 2003
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Within this new framework, how to approach cell
function?
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Do gels act?

• phase-transition

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gel
phase-transition is a critical phenomenon
temperature, solvent composition, pH, ions,
electric field, UV light, specific molecules or
chemicals
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How does the phase-transition work?
Polymer-polymer affinity dominates Condensed
Polymer-water affinity dominates EXPANDED
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Divalent cations induce condensation
Negatively charged sites
(reversed by monovalent cations)
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Negatively charged polymer
divalent cations
divalent
Polymer moves Water moves Ions move
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Reverse

Na

K
divalent cations
(Divalent cations released)
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Isotropic
Linear
Divalent cations cross-link polymer
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Hypothesis cell action occurs by
phase-transitions
(i.e., phase-transition within each organelle)
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Processes considered today

• Secretion
• action potential
• intracellular transport
• contraction
• cell motility
• ciliary and flagellar action
• mitosis
• transmembrane transport

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Problem Molecules trapped in network
Textbook mechanism
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How can molecules exit?

• phase transition

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Proposed mechanism
(After P. Verdugo, J. Fernandez)
Na
Na
Na
Na
Na
Na
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Action potential
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Conventional view(Na channels K channels)
K out
Na in

• Remove outside sodium action potentials remain
• Remove inside potassium action potentials remain

(numerous papers)
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-
Na penetrates into condensed gel
Another phase transition?
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Tasaki NIH, Matsumoto Japan
Peripheral cytoskeleton
condensed state
trigger
20 mV
unzip water entry increased permeability more
sodium entry positive swing of potential
- 60 mV
network collapses initial condition reset
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Linearly oriented polymers
Finally

• Actin filaments (streaming)
• Muscle contraction

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Cell movement
Internal mass movement
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Basis
myosin monomers
miscellaneous
Phase-transition mechanism?
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Phase transition transports solutes Zone
refining
silicon, or germanium
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Similar principle along actin bundle?

• Requirement 1 local phase change

Requirement 2 propagation
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Evidence for structural change in actin

• Length changes in streaming bundles, 15
  (Kamiya)
• Actin intra-monomer spacing changes 17 -
  fluorescence energy transfer (Miki Koyama)
• Actin repeat changes by 15 - X-ray diffraction
  of actin-profilin crystals (Schutt and Lindberg)
• Monomers rotate on myosin exposure -
  fluorescence polarization (Yanagida Oosawa)

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Evidence for propagation (1)
gelsolin
(Prochniewicz et al., 1996)
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Actin filament translating over a bed of myosin
Evidence for propagation (2)
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markers
Hatori et al. (1996, 1998)
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Muscle Contraction
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Contractile filaments
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Z
Z
cf. Iwazumi, Schutt and Lindberg, Oplatka,
Pollack
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markers
Propagating phase-change mechanism?
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.
Z
.
Z
.
Z
Expectations one step per wave step size
n x 2.7 nm
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(Liu and Pollack, Biophys J. 2003)
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Single sarcomere
10 nm
1 sec
(Yakovenko et al., 2002)
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Conclusion

• Muscle contraction could well involve a
  phase-transition propagating along the actin
  filament

(the same propagating transition that drives
streaming)
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Phase-transitions implicated in

• secretion
• action potential
• intracellular transport
• contraction
• cell motility
• ciliary and flagellar action
• mitosis
• transmembrane transport

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How do cells work?
Phase-transition
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A 305 page preface to the future of cell
biology Cell, July 2002
www.cellsandgels.com
info