221 research outputs found
The role of body rotation in bacterial flagellar bundling
In bacterial chemotaxis, E. coli cells drift up chemical gradients by a
series of runs and tumbles. Runs are periods of directed swimming, and tumbles
are abrupt changes in swimming direction. Near the beginning of each run, the
rotating helical flagellar filaments which propel the cell form a bundle. Using
resistive-force theory, we show that the counter-rotation of the cell body
necessary for torque balance is sufficient to wrap the filaments into a bundle,
even in the absence of the swirling flows produced by each individual filament
Twirling Elastica: Kinks, Viscous Drag, and Torsional Stress
Biological filaments such as DNA or bacterial flagella are typically curved
in their natural states. To elucidate the interplay of viscous drag, twisting,
and bending in the overdamped dynamics of such filaments, we compute the
steady-state torsional stress and shape of a rotating rod with a kink. Drag
deforms the rod, ultimately extending or folding it depending on the kink
angle. For certain kink angles and kink locations, both states are possible at
high rotation rates. The agreement between our macroscopic experiments and the
theory is good, with no adjustable parameters.Comment: 4 pages, 4 figure
Beating patterns of filaments in viscoelastic fluids
Many swimming microorganisms, such as bacteria and sperm, use flexible
flagella to move through viscoelastic media in their natural environments. In
this paper we address the effects a viscoelastic fluid has on the motion and
beating patterns of elastic filaments. We treat both a passive filament which
is actuated at one end, and an active filament with bending forces arising from
internal motors distributed along its length. We describe how viscoelasticity
modifies the hydrodynamic forces exerted on the filaments, and how these
modified forces affect the beating patterns. We show how high viscosity of
purely viscous or viscoelastic solutions can lead to the experimentally
observed beating patterns of sperm flagella, in which motion is concentrated at
the distal end of the flagella
Network development in biological gels: role in lymphatic vessel development
In this paper, we present a model that explains the prepatterning of lymphatic vessel morphology in collagen gels. This model is derived using the theory of two phase rubber material due to Flory and coworkers and it consists of two coupled fourth order partial differential equations describing the evolution of the collagen volume fraction, and the evolution of the proton concentration in a collagen implant; as described in experiments of Boardman and Swartz (Circ. Res. 92, 801–808, 2003). Using linear stability analysis, we find that above a critical level of proton concentration, spatial patterns form due to small perturbations in the initially uniform steady state. Using a long wavelength reduction, we can reduce the two coupled partial differential equations to one fourth order equation that is very similar to the Cahn–Hilliard equation; however, it has more complex nonlinearities and degeneracies. We present the results of numerical simulations and discuss the biological implications of our model
Twirling and Whirling: Viscous Dynamics of Rotating Elastica
Motivated by diverse phenomena in cellular biophysics, including bacterial
flagellar motion and DNA transcription and replication, we study the overdamped
nonlinear dynamics of a rotationally forced filament with twist and bend
elasticity. Competition between twist injection, twist diffusion, and writhing
instabilities is described by a novel pair of coupled PDEs for twist and bend
evolution. Analytical and numerical methods elucidate the twist/bend coupling
and reveal two dynamical regimes separated by a Hopf bifurcation: (i)
diffusion-dominated axial rotation, or twirling, and (ii) steady-state
crankshafting motion, or whirling. The consequences of these phenomena for
self-propulsion are investigated, and experimental tests proposed.Comment: To be published in Physical Review Letter
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Cyclin-dependent kinase control of motile ciliogenesis
Cycling cells maintain centriole number at precisely two per cell in part by limiting their duplication to S phase under the control of the cell cycle machinery. In contrast, postmitotic multiciliated cells (MCCs) uncouple centriole assembly from cell cycle progression and produce hundreds of centrioles in the absence of DNA replication to serve as basal bodies for motile cilia. Although some cell cycle regulators have previously been implicated in motile ciliogenesis, how the cell cycle machinery is employed to amplify centrioles is unclear. We use transgenic mice and primary airway epithelial cell culture to show that Cdk2, the kinase responsible for the G1 to S phase transition, is also required in MCCs to initiate motile ciliogenesis. While Cdk2 is coupled with cyclins E and A2 during cell division, cyclin A1 is required during ciliogenesis, contributing to an alternative regulatory landscape that facilitates centriole amplification without DNA replication
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
Differential expression of members of the E2F family of transcription factors in rodent testes
BACKGROUND: The E2F family of transcription factors is required for the activation or repression of differentially expressed gene programs during the cell cycle in normal and abnormal development of tissues. We previously determined that members of the retinoblastoma protein family that interacts with the E2F family are differentially expressed and localized in almost all the different cell types and tissues of the testis and in response to known endocrine disruptors. In this study, the cell-specific and stage-specific expression of members of the E2F proteins has been elucidated. METHODS: We used immunohistochemical (IHC) analysis of tissue sections and Western blot analysis of proteins, from whole testis and microdissected stages of seminiferous tubules to study the differential expression of the E2F proteins. RESULTS: For most of the five E2F family members studied, the localizations appear conserved in the two most commonly studied rodent models, mice and rats, with some notable differences. Comparisons between wild type and E2F-1 knockout mice revealed that the level of E2F-1 protein is stage-specific and most abundant in leptotene to early pachytene spermatocytes of stages IX to XI of mouse while strong staining of E2F-1 in some cells close to the basal lamina of rat tubules suggest that it may also be expressed in undifferentiated spermatogonia. The age-dependent development of a Sertoli-cell-only phenotype in seminiferous tubules of E2F-1 knockout males corroborates this, and indicates that E2F-1 is required for spermatogonial stem cell renewal. Interestingly, E2F-3 appears in both terminally differentiated Sertoli cells, as well as spermatogonial cells in the differentiative pathway, while the remaining member of the activating E2Fs, E2F-2 is most concentrated in spermatocytes of mid to late prophase of meiosis. Comparisons between wildtype and E2F-4 knockout mice demonstrated that the level of E2F-4 protein displays a distinct profile of stage-specificity compared to E2F-1, which is probably related to its prevalence and role in Sertoli cells. IHC of rat testis indicates that localization of E2F-5 is distinct from that of E2F-4 and overlaps those of E2F-1 and E2F-2. CONCLUSION: The E2F-1 represents the subfamily of transcription factors required during stages of DNA replication and gene expression for development of germ cells and the E2F-4 represents the subfamily of transcription factors that help maintain gene expression for a terminally differentiated state within the testis
Motor-Driven Bacterial Flagella and Buckling Instabilities
Many types of bacteria swim by rotating a bundle of helical filaments also
called flagella. Each filament is driven by a rotary motor and a very flexible
hook transmits the motor torque to the filament. We model it by discretizing
Kirchhoff's elastic-rod theory and develop a coarse-grained approach for
driving the helical filament by a motor torque. A rotating flagellum generates
a thrust force, which pushes the cell body forward and which increases with the
motor torque. We fix the rotating flagellum in space and show that it buckles
under the thrust force at a critical motor torque. Buckling becomes visible as
a supercritical Hopf bifurcation in the thrust force. A second buckling
transition occurs at an even higher motor torque. We attach the flagellum to a
spherical cell body and also observe the first buckling transition during
locomotion. By changing the size of the cell body, we vary the necessary thrust
force and thereby obtain a characteristic relation between the critical thrust
force and motor torque. We present a sophisticated analytical model for the
buckling transition based on a helical rod which quantitatively reproduces the
critical force-torque relation. Real values for motor torque, cell body size,
and the geometry of the helical filament suggest that buckling should occur in
single bacterial flagella. We also find that the orientation of pulling
flagella along the driving torque is not stable and comment on the biological
relevance for marine bacteria.Comment: 15 pages, 11 figure
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