11,537 research outputs found
Possible mechanisms for initiating macroscopic left-right asymmetry in developing organisms
How might systematic left-right (L/R) asymmetry of the body plan originate in
multicellular animals (and plants)? Somehow, the microscopic handedness of
biological molecules must be brought up to macroscopic scales. Basic symmetry
principles suggest that the usual "biological" mechanisms -- diffusion and gene
regulation -- are insufficient to implement the "right-hand rule" defining a
third body axis from the other two. Instead, on the cellular level, "physical"
mechanisms (forces and collective dynamic states) are needed involving the long
stiff fibers of the cytoskeleton. I discuss some possible scenarios; only in
the case of vertebrate internal organs is the answer currently known (and even
that is in dispute).Comment: 9 pp latex, 6 figures. Proc. Landau 100 Memorial Conf.
(Chernogolovka, June 2008); to appear AIP Conf. series. (v2: added 4 ref's +
revised Sec 2.2.
Possible origins of macroscopic left-right asymmetry in organisms
I consider the microscopic mechanisms by which a particular left-right (L/R)
asymmetry is generated at the organism level from the microscopic handedness of
cytoskeletal molecules. In light of a fundamental symmetry principle, the
typical pattern-formation mechanisms of diffusion plus regulation cannot
implement the "right-hand rule"; at the microscopic level, the cell's
cytoskeleton of chiral filaments seems always to be involved, usually in
collective states driven by polymerization forces or molecular motors. It seems
particularly easy for handedness to emerge in a shear or rotation in the
background of an effectively two-dimensional system, such as the cell membrane
or a layer of cells, as this requires no pre-existing axis apart from the layer
normal. I detail a scenario involving actin/myosin layers in snails and in C.
elegans, and also one about the microtubule layer in plant cells. I also survey
the other examples that I am aware of, such as the emergence of handedness such
as the emergence of handedness in neurons, in eukaryote cell motility, and in
non-flagellated bacteria.Comment: 42 pages, 6 figures, resubmitted to J. Stat. Phys. special issue.
Major rewrite, rearranged sections/subsections, new Fig 3 + 6, new physics in
Sec 2.4 and 3.4.1, added Sec 5 and subsections of Sec
A model for hand-over-hand motion of molecular motors
A simple flashing ratchet model in two dimensions is proposed to simulate the
hand-over-hand motion of two head molecular motors like kinesin. Extensive
Langevin simulations of the model are performed. Good qualitative agreement
with the expected behavior is observed. We discuss different regimes of motion
and efficiency depending of model parameters.Comment: 8 pages, Phys. Rev. E (in press
Cytoskeleton and Cell Motility
The present article is an invited contribution to the Encyclopedia of
Complexity and System Science, Robert A. Meyers Ed., Springer New York (2009).
It is a review of the biophysical mechanisms that underly cell motility. It
mainly focuses on the eukaryotic cytoskeleton and cell-motility mechanisms.
Bacterial motility as well as the composition of the prokaryotic cytoskeleton
is only briefly mentioned. The article is organized as follows. In Section III,
I first present an overview of the diversity of cellular motility mechanisms,
which might at first glance be categorized into two different types of
behaviors, namely "swimming" and "crawling". Intracellular transport, mitosis -
or cell division - as well as other extensions of cell motility that rely on
the same essential machinery are briefly sketched. In Section IV, I introduce
the molecular machinery that underlies cell motility - the cytoskeleton - as
well as its interactions with the external environment of the cell and its main
regulatory pathways. Sections IV D to IV F are more detailed in their
biochemical presentations; readers primarily interested in the theoretical
modeling of cell motility might want to skip these sections in a first reading.
I then describe the motility mechanisms that rely essentially on
polymerization-depolymerization dynamics of cytoskeleton filaments in Section
V, and the ones that rely essentially on the activity of motor proteins in
Section VI. Finally, Section VII is devoted to the description of the
integrated approaches that have been developed recently to try to understand
the cooperative phenomena that underly self-organization of the cell
cytoskeleton as a whole.Comment: 31 pages, 16 figures, 295 reference
Intracellular transport driven by cytoskeletal motors: General mechanisms and defects
Cells are strongly out-of-equilibrium systems driven by continuous energy
supply. They carry out many vital functions requiring active transport of
various ingredients and organelles, some being small, others being large. The
cytoskeleton, composed of three types of filaments, determines the shape of the
cell and plays a role in cell motion. It also serves as a road network for the
so-called cytoskeletal motors. These molecules can attach to a cytoskeletal
filament, perform directed motion, possibly carrying along some cargo, and then
detach. It is a central issue to understand how intracellular transport driven
by molecular motors is regulated, in particular because its breakdown is one of
the signatures of some neuronal diseases like the Alzheimer.
We give a survey of the current knowledge on microtubule based intracellular
transport. We first review some biological facts obtained from experiments, and
present some modeling attempts based on cellular automata. We start with
background knowledge on the original and variants of the TASEP (Totally
Asymmetric Simple Exclusion Process), before turning to more application
oriented models. After addressing microtubule based transport in general, with
a focus on in vitro experiments, and on cooperative effects in the
transportation of large cargos by multiple motors, we concentrate on axonal
transport, because of its relevance for neuronal diseases. It is a challenge to
understand how this transport is organized, given that it takes place in a
confined environment and that several types of motors moving in opposite
directions are involved. We review several features that could contribute to
the efficiency of this transport, including the role of motor-motor
interactions and of the dynamics of the underlying microtubule network.
Finally, we discuss some still open questions.Comment: 74 pages, 43 figure
Brownian molecular motors driven by rotation-translation coupling
We investigated three models of Brownian motors which convert rotational
diffusion into directed translational motion by switching on and off a
potential. In the first model a spatially asymmetric potential generates
directed translational motion by rectifying rotational diffusion. It behaves
much like a conventional flashing ratchet. The second model utilizes both
rotational diffusion and drift to generate translational motion without spatial
asymmetry in the potential. This second model can be driven by a combination of
a Brownian motor mechanism (diffusion driven) or by powerstroke (drift driven)
depending on the chosen parameters. In the third model, elements of both the
Brownian motor and powerstroke mechanisms are combined by switching between
three distinct states. Relevance of the model to biological motor proteins is
discussed.Comment: 11 pages, 8 figure
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