Biology subjects

Gholami, A.

Publications and source records attributed to Gholami, A..

6 recordsLinked to original sources

bioRxiv2021

Anomalous propulsion regime in axonemal-propelled cargoes

Bio-hybrid micro-swimmers, composed of biological entities integrated with synthetic constructs, actively transport cargo by converting chemical energy into mechanical work in a fluid at low Reynolds number, where viscous drag dominates over inertia. Here, using isolated and demembranated flagella from green algae Chlamydomonas reinhardtii (C. reinhardtii), we build efficient axonemally-driven micro-swimmers that consume ATP to propel micron-sized beads. Depending on the calcium concentration, we observed two main classes of motion: Whereas beads move along curved trajectories at calcium concentrations below 0.03 mM, they are propelled along straight paths when the calcium concentration increases. In this regime, they reached velocities of approximately 20 m/sec, comparable to human sperm velocity in vivo. We relate this transition to the properties of beating axonemes, in particular the reduced static curvature with increasing calcium concentration. To quantify the motion, we used mode decomposition of the flagellar waveform, and we studied both analytically and numerically the propulsion of the bead as a function of the axonemal waveform and bead-axoneme attachment geometry. While our analysis semi-quantitatively describes the experimental results, it also reveals the existence of a counter-intuitive propulsion regime where the speed of the axonemally-driven bead increases with the size of the bead. Moreover, we demonstrated that asymmetric, sideways attachment of the axoneme to the bead can also contribute to the rotational velocity of the micro-swimmer. The uncovered mechanism has potential applications in the fabrication of synthetic micro-swimmers, and in particular, bio-actuated medical micro-robots for targeted drug delivery.

biophysics↗

bioRxiv2021

Time-reversal symmetric component of the flagellar beat enhances the translational and rotational velocities of isolated Chlamydomonas flagella

The beating of cilia and flagella is essential to perform many important biological functions, including generating fluid flows on the cell surface or propulsion of micro-organisms. In this work, we analyze the motion of isolated and demembranated flagella from green algae Chlamydomonas reinhardtii, which act as ATP-driven micro-swimmers. The waveform of the Chlamydomonas beating flagella has an asymmetric waveform that is known to involve the superposition of a static component, corresponding to a fixed, intrinsic curvature, and a dynamic wave component traveling in the base-to-tip direction at the fundamental beat frequency, plus higher harmonics. Here, we demonstrate that these modes are not sufficient to reproduce the observed flagella waveforms. We find that two extra modes play an essential role to describe the motion: first, a time-symmetric mode, which corresponds to a global oscillation of the axonemal curvature, and second, a secondary tip-to-base wave component at the fundamental frequency that propagates opposite to the dominant base-to-tip wave, albeit with a smaller amplitude. Although the time-symmetric mode cannot, by itself, contribute to propulsion (scallop theorem), it does enhance the translational and rotational velocities of the flagellum by approximately a factor of 2. This mode highlights a long-range coupled on/off activity of force-generating dynein motors and can provide further insight into the underling biology of the ciliary beat.

biophysics↗

bioRxiv2020

Resistive force theory and wave dynamics in swimming isolated flagellar apparatus

Cilia-driven motility and fluid transport is ubiquitous in nature and essential for many biological processes, including swimming of eukaryotic unicellular organisms, mucus transport in airway apparatus or fluid flow in brain. The-biflagellated micro-swimmer Chlamydomonas reinhardtii is a model organism to study dynamics of flagellar synchronization. Hydrodynamic interactions, intracellular mechanical coupling or cell body rocking are believed to play crucial role in synchronization of flagellar beating in green algae. Here, we use freely swimming intact flagellar apparatus isolated from wall-less strain of Chlamydomonas to investigate wave dynamics. Our analysis in phase coordinates show that, when the frequency difference between the flagella is high, neither mechanical coupling via basal body nor hydrodynamics interactions are strong enough to synchronize two flagella, indicating that beating frequency is controlled internally by the cell. We also examined the validity of resistive force theory for a flagellar apparatus swimming freely in the vicinity of a substrate and found a quantitative agreement between experimental data and simulations with drag anisotropy of ratio 2. Finally, using a simplified wave form, we investigated the influence of phase and frequency differences, intrinsic curvature and wave amplitude on the swimming trajectory of flagellar apparatus. Our analysis shows that by controlling phase or frequency differences between two flagella, steering can occur.

biophysics↗

bioRxiv2020

Three-dimensional beating dynamics of Chlamydomonas flagella

Axonemes are the basic structure of motile cilia and flagella, and the investigation of how they function and move requires rapid three-dimensional imaging. We built a multi-plane phase-contrast microscope for imaging the three-dimensional motion of unlabeled flagella of the model organism Chlamydomonas reinhardtii with sub-m spatial and 4 ms temporal resolution. This allows us to observe not only bending but also the three-dimensional torsional dynamics of these small structures. We observe that flagella swim counter-clockwise close to a surface, with negatively-valued torsion at their basal and positively-valued torsion at their distal tips. To explain the torsional dynamics and signature, we suggest the existence of an intrinsic negative twist at the basal end that is untwisted by active positive-twist-inducing dynein motor proteins. Moreover, dyneins walking towards the basal induce an opposite twist at the distal tip. Bending of the whole axoneme structure then translates this twist into an observable torsion. This interconnection between chiral structure, twist, curvature, and torsion is fundamental for understanding flagellar mechanics.

biophysics↗

bioRxiv2020

Light-powered reactivation of flagella: towards self-sustained movement of artificial cells

Artificial systems capable of self-sustained movement with self-sufficient energy are of high interest with respect to the development of many challenging applications including medical treatments but also technical applications. The bottom-up assembly of such systems in the context of synthetic biology is still a challenging task. In this work, we demonstrate the biocompatibility and efficiency of an artificial light-driven energy module and a motility functional unit by integrating light-switchable photosynthetic vesicles with demembranated flagella that provide ATP for dynein molecular motors upon illumination. The flagellar propulsion is coupled to the beating frequency and dynamic ATP synthesis in response to illumination allows us to control beating frequency of flagella in a light-dependent manner. In addition, we verified the functionality of light-powered synthetic vesicles in in vitro motility assays by encapsulating microtubules assembled with force-generating kinesin-1 motors and the energy module to investigate the dynamics of a contractile filamentous network in cell-like compartments by optical stimulation. Integration of this photosynthetic system with various biological building blocks such as cytoskeletal filaments and molecular motors may contribute to the bottom-up synthesis of artificial cells that are able to undergo motor-driven morphological deformations and exhibit directional motion in a light-controllable fashion. Graphical TOC Entry O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=82 SRC="FIGDIR/small/212191v3_ufig1.gif" ALT="Figure 1"> View larger version (20K): org.highwire.dtl.DTLVardef@79347aorg.highwire.dtl.DTLVardef@164463dorg.highwire.dtl.DTLVardef@3f8b01org.highwire.dtl.DTLVardef@1bef3e8_HPS_FORMAT_FIGEXP M_FIG C_FIG

biophysics↗

bioRxiv2019

Boundary-Driven Oscillations Rescue PdsA- cells

Dictyostelium discoideum amoeba aggregate if deprived of nutrients, producing cAMP waves at precisely timed intervals. Degradation of extracellular cAMP by the enzyme phosphodiesterase PdsA is fundamental to successfully producing waves, regulating the external cAMP gradient field and preventing the accumulation of cAMP. The knockout mutant PdsA- produces no or a greatly reduced amount of main extracellular phosphodiesterase, therefore failing to relay cAMP waves and aggregate under starvation conditions. Using a microfluidic channel, we show how an advective flow can partially recover signaling in a population of starving PdsA- cells. Above a minimum flow velocity, decaying waves are induced, with a decay length that increases with the imposed flow velocity. Interestingly, after stopping the advecting flow, the cells continue to signal, showing wave propagation and aggregation, although with a wave period much higher than in wild type cells. We performed extensive numerical simulations and showed that these waves have a boundary-driven origin, where the lack of cAMP in the upstream flow destabilizes the system. We explored the properties of these waves and the parameter region where they exist, with good agreement with our experimental observations. These boundary-driven waves dominate the system dynamics in the velocity range where they exist, while at higher flow velocities the natural wave period of 6 min recovers. These results provide experimental confirmation of the destabilizing effect of the upstream boundary in an otherwise stable reaction-diffusion system. We expect this mechanism to be relevant for wave creation in other oscillatory or excitable systems that are incapable of normal pattern formation.\n\nSIGNIFICANCE STATEMENTWe present experimental evidence for the existence of boundary-driven instabilities in a reaction-diffusion-advection system. In our theoretical prediction (1), we have shown that imposing an absorbing boundary condition on the upstream end of a flow-through channel filled with signaling cells creates an instability capable of periodically producing wave trains which are advected downstream. Under starvation, these cells secret the signaling molecule cAMP as well as the degrading agent phosphodiestrase that degrades cAMP. This instability was predicted to exist at lower degradation rates of cAMP and thus was expected to provide a mechanism for wave creation in phosphodiesterase deficient systems, such as PdsA- cells. Our experiments confirm the importance of the upstream boundary condition and show that boundary-driven oscillations are relevant in reaction-diffusion systems.

biophysics↗