Office: BSB 426; Telephone: (856) 225-6159; fax (856) 225-6624
Office Hours: T, Th 8:00-9:20 AM
gagliard@camden.rutgers.edu
Courses Taught (Fall 2008):
Electric Circuits I
[1.]
Microscale Electrostatics in Mitosis Journal of Electrostatics,
vol. 54, 210-232 (2002).
Comments on current research
In spite of a series of ingenious experiments over the last 20 - 25 years,
the cause for many of the mitotic
motions remains unclear. The
expectation of many researchers in the field has been that
molecular motors (MMs) will eventually be shown to be the motive force for
chromosome (CHR) motions. This may be changing. Several new non-MM models
have been proposed recently that are either fundamentally electrostatic
in nature (although not explicitly stated)
or leave the precise mechanism for force generation unspecified in
a simulation that introduces a number of adjustable parameters.
The
current paradigm of molecular biology requires that specific molecules,
or molecular geometries, for force generation be identified. However, it
is possible to account for
the forces and mechanisms for mitotic motions in terms of
experimentally known cellular electric charge distributions interacting
over nanometer distances without having specific molecules in mind.
This is the approach that I have
taken in the series of papers listed here. A crude
analogy can be seen in the well-known parlor trick of attracting bits of
paper with a charged comb. This motion is simply explained in terms of the
known negative charge on the comb attracting the nearer positive charge on
the bits of paper with the bits of paper remaining overall electrically
neutral. There is no need to identify the specific molecules in the paper
(or in the comb for that matter) that are responsible for the attractive
force and the subsequent motion. In fact, since the charged comb will
attract many different kinds of paper as well as many other substances,
attempting to identify the molecules in the paper or in the various
substances is counterproductive to explaining what is happening.
Similarly, CHR motions can be described in terms
of electric charge distributions. The known charge in mitotic
CHR motions is the net charge at the plus and minus ends of
MTs. A major advantage of this
approach 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.
As mentioned above, several new,
fundamentally electrostatic, models have been advanced recently involving
particular suspected molecules and their geometrical interactions. As in
the situation involving models that center partially or wholly on
simulations,
these approaches are quite complex in their assumptions and attempt to
explain only poleward
generation of force at kinetochores. This is to be contrasted with the
minimal assumptions comprehensive model for post-attachment CHR
motions presented in the series outlined here.
It is thought that MMs are likely to be
involved in the sliding microtubule sidewall "capture" motion of
chromatid
pairs. The case for MMs as the cause for post-attachment
CHR motions is considerably less convincing.
There are substantive arguments against MMs as the cause for
post-attachment prometaphase, metaphase, and anaphase CHR motions.
Among these are:
Anaphase-A has been observed to proceed in isolated spindles in the
absence of ATP if conditions in the experimental system are set up to
promote MT disassembly [a].
Observations of MT flux at poles accompanying CHR
poleward movement would seem to be most consistently explained by force
generation at cell poles, yet there does not appear to be much
in the literature regarding specific models
for force generation by MMs at spindle poles.
The slowing or stopping of anaphase-A CHR motion with calcium ion
concentration increases slightly above the optimum concentration for
maximum
CHR speed - but far from concentrations that compromise the
mitotic spindle - is difficult to address within a model based
on MMs [b,c].
In a key experiment with grasshopper spermatocytes [d] it was found
that both anaphase-A and -B proceeded independently of CHRs. The
authors of this study concluded that CHRs, when present, might
migrate to the poles during anaphase-A by having their kinetochores latch
onto the ends of
shortening MTs. Why would cells need to utilize MMs when the
requisite motion for anaphase-A CHR separation is already present?
[a]. D.E. Koshland, T.J. Mitchison, and M.W. Kirshner, Nature, vol.331
(1988) 499.
[b]. D.H. Zhang, D.A. Callaham, and P.K. Hepler, J. Cell Biol., vol. 111
(1990) 171.
[c]. S.L. Wolfe, Molecular and Cellular Biology, 2nd ed., Wadsworth,
Belmont, CA. (1993), p. 425.
[d]. D. Zhang and R.B. Nicklas, Nature, vol. 382 (1996) 466.
As discussed above, identifying the motive force is central to explaining
CHR motions
during mitosis. Presently, there is no consensus on what it is. I have
proposed a minimal assumptions model for the dynamics of
post-attachment CHR motions based on nanoscale electrostatics.
Given the
electrical properties of tubulin and the dynamic instability of
MTs it is possible to account for prometaphase post-attachment,
metaphase, and anaphase CHR motions within a comprehensive model.
MT subunits are electric dipolar structures
that can act as intermediaries extending the reach of electrostatic
interactions in spite of counter-ion screening. This allows prometaphase
post-attachment and metaphase CHR movements to be explained by
(1) antipoleward nanoscale electrostatic
repulsive MT assembly forces acting between the negatively
charged free ends of astral
MTs and CHR arms, combined with (2)
poleward-directed nanoscale electrostatic kinetochore
MT disassembly forces acting at kinetochores and spindle poles.
After a bivalent attachment to both poles has been completed, an
approximate "inverse
square" distance dependence of the electrostatic antipoleward force acting
at CHR arms ensures congressional motion of chromatid pairs to the
midcell region. Stable metaphase mid-cell oscillatory motion is a direct
consequence of the inverse square dependence of the antipoleward forces.
With the experimentally observed increase in calcium ion concentration at
the onset of anaphase-A, the probability for MT assembly is
decreased significantly, allowing electrostatic poleward MT
disassembly
forces acting at kinetochores and poles to dominate, and anaphase-A motion
ensues. Anaphase-B cell elongation is addressed consistently by
electrostatic repulsion between
adjacent negatively charged plus ends of polar
MTs originating from opposite poles as subsets of these
MTs disassemble at plus ends while
their minus ends preferentially assemble at poles.
This combination decreases the amount of MT overlap while
keeping
relatively constant average half-spindle MT lengths,
resulting in cell elongation.
The present model addresses all of the following in a unified
manner (See above links to numbered references. Page numbers in ref's. 2
and 4 refer to the preprint versions linked to
this website and differ from the
actual page numbers in the Physical Review E and Journal of
Electrostatics papers):
1. Efficiency of aster and spindle assembly and the motive force for the
motion of asters ([1], p.222).
2. Chromatid pair attachment ([2], p. 12).
3. Motion of monovalently attached chromatid pairs ([2], pp. 12 & 13; [3],
pp. 320 & 321).
4. Motion of bivalently attached chromatid pairs; congression ([2], pp. 13
& 14; [3], pp. 321 & 322).
5. Metaphase chromatid pair oscillations ([2], pp. 14 & 15; [3], p. 322).
6. Anaphase-A CHR motion ([3], pp. 322 & 323; [4], pp. 19-21).
7. Anaplase-B pole separation ([1], pp. 228-230; [4], pp. 21 & 22).
8. An ab initio theoretical calculation of the maximum force exerted
by a MT during mitosis that falls
within the experimental range ([3], pp. 315-318; [4],
pp.16-18).
9. Poleward force generation at poles with associated MT flux
is explained by the same kinetochore MT nanoscale
electrostatic disassembly force as at kinetochores [3], [4].