Telephone: (856) 489-4057
gagliard@camden.rutgers.edu
[4.] Microscale Electrostatics in Mitosis, Journal of Electrostatics,
vol. 54, 210-232 (2002b).
Comments on current research
In spite of a series of ingenious experiments over the last 20 - 25 years,
the cause for most of the mitotic
motions remains unclear. The
expectation of many researchers in the field had been that molecular
motors would eventually be shown to be the motive force for
chromosome motions. This may be changing. Several new models
have been proposed recently that are either electrostatic in nature
or leave the precise mechanism for force generation unspecified
in a simulation that introduces a number of adjustable parameters.
The current paradigm in molecular biology requires that specific
molecules, and/or
molecular geometries, for force generation be identified. However, it
is possible to account for mitotic motions in terms of
experimentally known cellular electric charge distributions interacting
over nanometer distances.
This is the approach that I have
taken in the series of papers listed here.
The known charge in mitotic
chromosome motions resides at kinetochores, centrosomes, and
on chromosome arms.
In 2002 and 2005 papers, I argued that indirect experimental evidence
indicates that
pole-facing "plates" of kinetochores manifest positive charge (see
references [2] and [3]
above). A number of recent experiments have now implicated
positively charged molecules in kinetochores in establishing a
dynamic coupling to microtubules for force generation during mitosis.
Assuming positive charge at kinetochores, and negative charge at
and near the free plus ends of microtubules, it is possible to derive a
magnitude of the maximum (tension) force per microtubule for
poleward chromosome motions that falls within the experimental range [2].
In the above-mentioned papers I also proposed that indirect experimental
evidence is
consistent with negative charge on centrosomes. Recent direct
experimental measurements have confirmed this [Hormeno S. et al.,
Biophys. J. 97 (2009) 1022].
A major advantage of focusing on known cellular charge distributions
is that it appears to offer the possibility of discovering a
minimal assumptions comprehensive model for post-attachment chromosome
motions.
A model of this sort can point the way to the eventual
discovery of specific molecules, and their biochemistries, that are
responsible for the various mitotic motions.
This is what is happening. As mentioned above, recent experiments
have shown that kinetochore molecules that bind with microtubules are
positively charged, and that force for chromosome motions
may be due to electrostatic interactions between these molecules and
negative charge on microtubules. These discoveries might have been made
sooner if the above mentioned papers had been duly noted.
The recent experimental verification of net
negative charge on centrosomes implies that expression of positive charge
on free minus microtubule ends will be favored. This is because
of the lower local pH vicinal to the centrosome matrix as well as positive
charge induction. Additionally, the beta monomers that crown minus ends
are known to contain considerable positive charge.
The interaction of this charge with a
negatively charged centrosome may be responsible for polar poleward force
generation. A calculation of the force per microtubule assuming permanent
charge falls within the experimental range [2].
Although a calculation of the induced charge on a
microtubule minus end is difficult because of the geometry, a reciprocal
calculation of the induced
charge on a centrosome matrix from charge at the free end
of a microtubule is relatively straightforward, yields the same order of
magnitude for the polar force per microtubule, and falls within the
experimental range [1].
As mentioned above, electrostatic models within the molecular biology
paradigm are being sought (or advanced) to explain recent experiments
suggesting that highly basic
kinetochore molecules interact with negative
charge on microtubules to couple kinetochores to microtubules.
As in the situation involving models that center partially or wholly on
simulations, these approaches are quite complex in their assumptions and
primarily attempt to address one or two mitotic motions, most notably
poleward force generation at kinetochores. Critical experimental
observations such as the "slip-clutch" mechanism [Maiato et al., 2004],
observations of calcium ion concentration on anaphase-A
chromosome motion [Zhang et al., 1990], and polar generation of force
for poleward chromosome motions are not addressed. In the comprehensive
model that I have proposed ([1], [2], [3], [4]; Gagliardi, 2009 (book)),
interactions between stably bound, volume
charge distributions can account for these, as well as poleward force
generation at kinetochores.
[ Maiato H., DeLuca J., Salmon E.D., Earnshaw W.C. 2004. J. Cell Science,
vol. 117, p. 5461. ]
Regarding molecular motor contributions to mitotic motions,
it is generally thought that molecular motors are likely to be
involved in the sliding microtubule sidewall "capture" motion of
chromatid pairs. The case for molecular motors as the cause for
post-attachment chromosome motions is considerably less convincing.
[ Zhang D.H., Callaham D.A., Hepler P.K. 1990. J. Cell Biol.,
vol. 111, p. 171. ]
As discussed above, identifying the motive force is central to explaining
chromosome motions
during mitosis. Presently, there is no consensus on what it is. I have
proposed a minimal assumptions model for
post-attachment chromosome movements based on the dynamic
instability of microtubules and nanoscale electrostatics.
Given the electrical properties of (tubulin and) microtubules, and
charge distributions on kinetochores, centrosome matrices and
chromosome arms,
as well as the pH-dependent dynamics of microtubules,
it is possible to account for prometaphase post-attachment,
metaphase, and anaphase chromosome motions within a comprehensive model
that addresses all of the following in a unified manner:
1. Efficiency of aster and spindle assembly and the motive force for the
motion of asters and forming spindles
( [4], p.222 ).*
*See above links to numbered references. Page numbers in references 1
and 3 refer to the preprint versions linked to
this website, and differ from the actual page numbers in the Journal
of Electrostatics and Physical Review E papers.
2. Chromatid pair attachment ( [3], p. 12 ).
3. Motion of monovalently attached chromatid pairs ( [2],
pp. 320 & 321; [3], pp. 12 & 13 ).
4. Motion of bivalently attached chromatid pairs; congression ( [2],
pp. 321 & 322; [3], pp. 13 & 14 ).
5. Metaphase chromatid pair oscillations ( [2], p. 322; [3], pp. 14 & 15
).
6. Anaphase-A chromosome motion ( [1], pp. 19-21; [2], pp. 322 & 323 ).
7. Anaphase-B pole separation ( [1], pp. 21 & 22; [4], pp. 228-230 ).
8. Poleward force generation at poles with associated microtubule flux
is explained by the same microtubule nanoscale
electrostatic disassembly force as at kinetochores ( [1], [2] ).
9. The timing and sequencing of the above motions ( [2]; [3]; book,
Chapter 5;
also see Chapter 5 Summary for a capsule version). My book offers
the latest on items 1 through 8.
10. An "ab initio" theoretical calculation
of the tension force exerted by a microtubule during mitosis
falls within the experimental range ( [1], pp. 16-18; [2], pp. 315-318 ).