Amitabha Nandi

Interplay of cell dynamics and epithelial tension during morphogenesis of the Drosophila pupal wing

How tissue shape emerges from the collective mechanical properties and behavior of individual cells is not understood. We combine experiment and theory to study this problem in the developing wing epithelium of Drosophila. At pupal stages, the wing-hinge contraction contributes to anisotropic tissue flows that reshape the wing blade. Here, we quantitatively account for this wing-blade shape change on the basis of cell divisions, cell rearrangements and cell shape changes. We show that cells both generate and respond to epithelial stresses during this process, and that the nature of this interplay specifies the pattern of junctional network remodeling that changes wing shape. We show that patterned constraints exerted on the tissue by the extracellular matrix are key to force the tissue into the right shape. We present a continuum mechanical model that quantitatively describes the relationship between epithelial stresses and cell dynamics, and how their interplay reshapes the wing. DOI: http://dx.doi.org/10.7554/eLife.07090.001

Regulated tissue fluidity steers zebrafish body elongation

The tailbud is the posterior leading edge of the growing vertebrate embryo and consists of motile progenitors of the axial skeleton, musculature and spinal cord. We measure the 3D cell flow field of the zebrafish tailbud and identify changes in tissue fluidity revealed by reductions in the coherence of cell motion without alteration of cell velocities. We find a directed posterior flow wherein the polarization between individual cell motion is high, reflecting ordered collective migration. At the posterior tip of the tailbud, this flow makes sharp bilateral turns facilitated by extensive cell mixing due to increased directional variability of individual cell motions. Inhibition of Wnt or Fgf signaling or cadherin 2 function reduces the coherence of the flow but has different consequences for trunk and tail extension. Modeling and additional data analyses suggest that the balance between the coherence and rate of cell flow determines whether body elongation is linear or whether congestion forms within the flow and the body axis becomes contorted.

Cell-Fibronectin Interactions Propel Vertebrate Trunk Elongation via Tissue Mechanics

During embryonic development and tissue homeostasis, cells produce and remodel the extracellular matrix (ECM). The ECM maintains tissue integrity and can serve as a substrate for cell migration. Integrin α5 (Itgα5) and αV (ItgαV) are the α subunits of the integrins most responsible for both cell adhesion to the ECM protein fibronectin (FN) and FN matrix fibrillogenesis. We perform a systems-level analysis of cell motion in the zebrafish tail bud during trunk elongation in the presence and absence of normal cell-FN interactions. Itgα5 and ItgαV have well-described roles in cell migration in vitro. However, we find that concomitant loss of itgα5 and itgαV leads to a trunk elongation defect without substantive alteration of cell migration. Tissue-specific transgenic rescue experiments suggest that the FN matrix on the surface of the paraxial mesoderm is required for body elongation via its role in defining tissue mechanics and intertissue adhesion.

Pivoting of microtubules around the spindle pole accelerates kinetochore capture

During cell division, spindle microtubules attach to chromosomes through kinetochores, protein complexes on the chromosome. The central question is how microtubules find kinetochores. According to the pioneering idea termed search-and-capture, numerous microtubules grow from a centrosome in all directions and by chance capture kinetochores. The efficiency of search-and-capture can be improved by a bias in microtubule growth towards the kinetochores, by nucleation of microtubules at the kinetochores and at spindle microtubules, by kinetochore movement, or by a combination of these processes. Here we show in fission yeast that kinetochores are captured by microtubules pivoting around the spindle pole, instead of growing towards the kinetochores. This pivoting motion of microtubules is random and independent of ATP-driven motor activity. By introducing a theoretical model, we show that the measured random movement of microtubules and kinetochores is sufficient to explain the process of kinetochore capture. Our theory predicts that the speed of capture depends mainly on how fast microtubules pivot, which was confirmed experimentally by speeding up and slowing down microtubule pivoting. Thus, pivoting motion allows microtubules to explore space laterally, as they search for targets such as kinetochores.

miRNA-regulated dynamics in circadian oscillator models

BackgroundWe have studied the dynamics of miRNA regulation in two models of circadian oscillators. miRNAs are a class of small RNA molecules (18–24 nucleotides) that are known to regulate gene expression at the post-transcriptional level by reducing the amount of proteins produced by translation. This is done either by blocking translation or by degradation of mRNAs, the latter being mainly due to the initiation of a set of processes induced by formation of the miRNA:mRNA complex. Although miRNAs are known to regulate a large number of fundamental biological processes such as growth and development, their role in the dynamics of regulation is not completely understood. In exceptional cases, in particular, they can also up-regulate gene expression.ResultsWe have studied simple biological systems wherein oscillations originate from negative auto regulation of gene expression. The regulation of gene expression by miRNAs is introduced into these models and the dynamics is studied via standard stochastic simulation techniques. We find that in addition to a reduction in the amplitude of the oscillation, inclusion of miRNAs in the models has the effect of altering the frequency of oscillation and thereby regulating the dynamics of protein production.ConclusionmiRNAs can have a profound effect on the dynamics of regulatory modules, both by control of amplitude, namely by affecting the level of gene expression, as well as by control or alteration of frequency, namely by interference with the temporal sequence of gene production or delivery. We believe that our results are valid for a variety of regulatory systems, beyond the exemplars discussed here.

Local Motion Analysis Reveals Impact of the Dynamic Cytoskeleton on Intracellular Subdiffusion

Intracellular transport is a complex interplay of ballistic transport along filaments and of diffusive motion, reliably delivering material and allowing for cell differentiation, migration, and proliferation. The diffusive regime, including subdiffusive, Brownian, and superdiffusive motion, is of particular interest for inferring information about the dynamics of the cytoskeleton morphology during intracellular transport. The influence of dynamic cytoskeletal states on intracellular transport are investigated in Dictyostelium discoideum cells by single particle tracking of fluorescent nanoparticles, to relate quantitative motion parameters and intracellular processes before and after cytoskeletal disruption. A local mean-square displacement (MSD) analysis separates ballistic motion phases, which we exclude here, from diffusive nanoparticle motion. In this study, we focus on intracellular subdiffusion and elucidate lag-time dependence, with particular focus on the impact of cytoskeleton compartments like microtubules and actin filaments. This method proves useful for binary motion state distributions. Experimental results are compared to simulations of a data-driven Langevin model with finite velocity correlations that captures essential statistical features of the local MSD algorithm. Specifically, the values of the mean MSD exponent and effective diffusion coefficients can be traced back to negative correlations of the motions increments. We clearly identify both microtubules and actin filaments as the cause for intracellular subdiffusion and show that actin-microtubule cross talk exerts viscosifying effects at timescales larger than 0.2 s. Our findings might give insights into material transport and information exchange in living cells, which might facilitate gaining control over cell functions.

APERIODIC NONCHAOTIC ATTRACTORS, STRANGE AND OTHERWISE

The main conceptual issues in the study of strange nonchaotic dynamics have been summarized and reviewed earlier [Prasad et al., 2001]. In the past five years, there has been further progress in the analysis of such attractors, and in understanding the nature of dynamical transitions in quasiperiodically forced systems. Here we discuss new developments which include the creation of strange nonchaotic attractors in excitable systems, the elucidation of the mechanisms for the intermittent and fractalization routes to SNA as well as their potential applications. The possibility of creating such attractors without recourse to quasiperiodic forcing is an important objective, and this has been realized in specific circumstances.

The phase-modulated logistic map

We study the logistic mapping with the nonlinearity parameter varied through a delayed feedback mechanism. This history dependent modulation through a phaselike variable offers an enhanced possibility for stabilization of periodic dynamics. Study of the system as a function of nonlinearity and modulation parameters reveals new phenomena: In addition to period-doubling and tangent bifurcations, there can be bifurcations where the period increases by unity. These are extensions of crises that arise in nonlinear dynamical systems. Periodic orbits in this system can be systematized via the kneading theory, which in the present case extends the analysis of Metropolis, Stein, and Stein for unimodal maps.

Active dynamics of tissue shear flow

We propose a hydrodynamic theory to describe shear flows in developing epithelial tissues. We introduce hydrodynamic fields corresponding to state properties of constituent cells as well as a contribution to overall tissue shear flow due to rearrangements in cell network topology. We then construct a constitutive equation for the shear rate due to topological rearrangements. We identify a novel rheological behaviour resulting from memory effects in the tissue. We show that anisotropic deformation of tissue and cells can arise from two distinct active cellular processes: generation of active stress in the tissue, and actively driven cellular rearrangements. These two active processes result in distinct cellular and tissue shape changes, depending on boundary conditions applied on the tissue. Our findings have consequences for the understanding of tissue morphogenesis during development.

Design strategies for the creation of aperiodic nonchaotic attractors.

Parametric modulation in nonlinear dynamical systems can give rise to attractors on which the dynamics is aperiodic and nonchaotic, namely, with largest Lyapunov exponent being nonpositive. We describe a procedure for creating such attractors by using random modulation or pseudorandom binary sequences with arbitrarily long recurrence times. As a consequence the attractors are geometrically fractal and the motion is aperiodic on experimentally accessible time scales. A practical realization of such attractors is demonstrated in an experiment using electronic circuits.