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The true slime mold Physarum polycephalum exhibits a vast array of sophisticated manipulations of its intracellular cytoplasm. Growing microplasmodia of Physarum have been observed to adopt an elongated tadpole shape, then contract in a rhythmic, traveling wave pattern that resembles peristaltic pumping. This contraction drives a fast flow of non-gelated cytoplasm along the cell longitudinal axis. It has been hypothesized that this flow of cytoplasm is a driving factor in generating motility of the plasmodium. In this work, we use two different mathematical models to investigate how peristaltic pumping within Physarum may be used to drive cellular motility. We compare the relative phase of flow and deformation waves predicted by both models to similar phase data collected from in vivo experiments using Physarum plasmodia. The first is a PDE model based on a dimensional reduction of peristaltic pumping within a finite length chamber. The second is a more sophisticated computational model which accounts for more general shape changes, more complex cellular mechanics, and dynamically modulated adhesion to the underlying substrate. This model allows us to directly compute cell crawling speed. Both models suggest that a mechanical asymmetry in the cell is required to reproduce the experimental observations. Such a mechanical asymmetry is also shown to increase the potential for cellular migration, as measured by both stress generation and migration velocity.

The true slime mold Physarum polycephalum exhibits a vast array of sophisticated manipulations of its intracellular cytoplasm. Growing microplasmodia of Physarum have been observed to adopt an elongated tadpole shape, then contract in a rhythmic, traveling wave pattern that resembles peristaltic pumping. This contraction drives a fast flow of non-gelated cytoplasm along the cell longitudinal axis. It has been hypothesized that this flow of cytoplasm is a driving factor in generating motility of the plasmodium. In this work, we use two different mathematical models to investigate how peristaltic pumping within Physarum may be used to drive cellular motility. We compare the relative phase of flow and deformation waves predicted by both models to similar phase data collected from in vivo experiments using Physarum plasmodia. The first is a PDE model based on a dimensional reduction of peristaltic pumping within a finite length chamber. The second is a more sophisticated computational model which accounts for more general shape changes, more complex cellular mechanics, and dynamically modulated adhesion to the underlying substrate. This model allows us to directly compute cell crawling speed. Both models suggest that a mechanical asymmetry in the cell is required to reproduce the experimental observations. Such a mechanical asymmetry is also shown to increase the potential for cellular migration, as measured by both stress generation and migration velocity.

The transport of cytoplasmic components can be profoundly affected by hydrodynamics. Cytoplasmic streaming in Drosophila oocytes offers a striking example. Forces on fluid from kinesin-1 are initially directed by a disordered meshwork of microtubules, generating minor slow cytoplasmic flows. Subsequently, to mix incoming nurse cell cytoplasm with ooplasm, a subcortical layer of microtubules forms parallel arrays that support long-range, fast flows. To analyze the streaming mechanism, we combined observations of microtubule and organelle motions with detailed mathematical modeling. In the fast state, microtubules tethered to the cortex form a thin subcortical layer and undergo correlated sinusoidal bending. Organelles moving in flows along the arrays show velocities that are slow near the cortex and fast on the inward side of the subcortical microtubule layer. Starting with fundamental physical principles suggested by qualitative hypotheses, and with published values for microtubule stiffness, kinesin velocity, and cytoplasmic viscosity, we developed a quantitative coupled hydrodynamic model for streaming. The fully detailed mathematical model and its simulations identify key variables that can shift the system between disordered (slow) and ordered (fast) states. Measurements of array curvature, wave period, and the effects of diminished kinesin velocity on flow rates, as well as prior observations on f-actin perturbation, support the model. This establishes a concrete mechanistic framework for the ooplasmic streaming process. The self-organizing fast phase is a result of viscous drag on kinesin-driven cargoes that mediates equal and opposite forces on cytoplasmic fluid and on microtubules whose minus ends are tethered to the cortex. Fluid moves toward plus ends and microtubules are forced backward toward their minus ends, resulting in buckling. Under certain conditions, the buckling microtubules self-organize into parallel bending arrays, guiding varying directions

Cytoplasmic streaming occurs in diverse cell types, where it generally serves a transport function. Here, we examine streaming in multicellular fungal hyphae and identify an additional function wherein regimented streaming forms distinct cytoplasmic subcompartments. In the hypha, cytoplasm flows directionally from cell to cell through septal pores. Using live-cell imaging and computer simulations, we identify a flow pattern that produces vortices (eddies) on the upstream side of the septum. Nuclei can be immobilized in these microfluidic eddies, where they form multinucleate aggregates and accumulate foci of the HDA-2 histone deacetylase-associated factor, SPA-19. Pores experiencing flow degenerate in the absence of SPA-19, suggesting that eddy-trapped nuclei function to reinforce the septum. Together, our data show that eddies comprise a subcellular niche favoring nuclear differentiation and that subcompartments can be self-organized as a consequence of regimented cytoplasmic streaming.

Background: The transport of cytoplasmic components can be profoundly affected by hydrodynamics. A striking example is ooplasmic streaming in Drosophila. Forces from kinesin-1 are initially directed by a disordered meshwork of oocyte microtubules, generating slow disordered cytoplasmic flows. When microtubules shift into parallel alignment, kinesin generates fast ordered flows that mix nurse cell and oocyte cytoplasms. Results: To understand the hydrodynamic mechanism of streaming, we used fluorescence microscopy to analyze microtubule organization and cytoplasmic flows, while using mathematical modeling to identify physical conditions that contribute to kinesin-driven self-organization. In the fast ordered state, microtubules align and undergo correlated bending in a subcortical layer. Cytoplasmic flows follow the curving microtubule paths with velocities that are slow near the cortex, faster within the microtubule layer, and fastest beneath it. FRAP and photoconversion indicate that minus-ends of the aligned microtubules are stationary relative to the cortex. Using known values for microtubule stiffness and kinesin velocity, we developed and tested a coupled hydrodynamic model that revealed key variables that can shift the system between disordered and ordered states, including: 1) the distance from the cortex at which microtubules can lie parallel to its plane, and 2) the intensity of kinesin force on its cortical microtubule tracks, which can be controlled by cytoplasmic viscosity.Conclusions: Cytoplasmic streaming in Drosophila oocytes is a result of viscous drag on moving kinesin motors that mediates equal and opposite forces on cytoplasmic fluid and on microtubules whose minus ends are embedded in the cortex. Fluid flows toward plus-ends and microtubules are forced toward stationary minus-ends. Under certain conditions, this causes microtubules to align in bending arrays, creating constantly varying directions of flow that facilitate cytoplasmic mixing

Background: The transport of cytoplasmic components can be profoundly affected by hydrodynamics. A striking example is ooplasmic streaming in Drosophila. Forces from kinesin-1 are initially directed by a disordered meshwork of oocyte microtubules, generating slow disordered cytoplasmic flows. When microtubules shift into parallel alignment, kinesin generates fast ordered flows that mix nurse cell and oocyte cytoplasms. Results: To understand the hydrodynamic mechanism of streaming, we used fluorescence microscopy to analyze microtubule organization and cytoplasmic flows, while using mathematical modeling to identify physical conditions that contribute to kinesin-driven self-organization. In the fast ordered state, microtubules align and undergo correlated bending in a subcortical layer. Cytoplasmic flows follow the curving microtubule paths with velocities that are slow near the cortex, faster within the microtubule layer, and fastest beneath it. FRAP and photoconversion indicate that minus-ends of the aligned microtubules are stationary relative to the cortex. Using known values for microtubule stiffness and kinesin velocity, we developed and tested a coupled hydrodynamic model that revealed key variables that can shift the system between disordered and ordered states, including: 1) the distance from the cortex at which microtubules can lie parallel to its plane, and 2) the intensity of kinesin force on its cortical microtubule tracks, which can be controlled by cytoplasmic viscosity.Conclusions: Cytoplasmic streaming in Drosophila oocytes is a result of viscous drag on moving kinesin motors that mediates equal and opposite forces on cytoplasmic fluid and on microtubules whose minus ends are embedded in the cortex. Fluid flows toward plus-ends and microtubules are forced toward stationary minus-ends. Under certain conditions, this causes microtubules to align in bending arrays, creating constantly varying directions of flow that facilitate cytoplasmic mixing

Cytoplasmic streaming occurs in diverse cell types, where it generally serves a transport function. Here, we examine streaming in multicellular fungal hyphae and identify an additional function wherein regimented streaming forms distinct cytoplasmic subcompartments. In the hypha, cytoplasm flows directionally from cell to cell through septal pores. Using live-cell imaging and computer simulations, we identify a flow pattern that produces vortices (eddies) on the upstream side of the septum. Nuclei can be immobilized in these microfluidic eddies, where they form multinucleate aggregates and accumulate foci of the HDA-2 histone deacetylase-associated factor, SPA-19. Pores experiencing flow degenerate in the absence of SPA-19, suggesting that eddy-trapped nuclei function to reinforce the septum. Together, our data show that eddies comprise a subcellular niche favoring nuclear differentiation and that subcompartments can be self-organized as a consequence of regimented cytoplasmic streaming.

This work examines the relationship between spatio-temporal coordination of intracellular flow and traction stress and the speed of amoeboid locomotion of microplasmodia of Physarum polycephalum. We simultaneously perform particle image velocimetry and traction stress microscopy to measure the velocity of cytoplasmic flow and the stresses applied to the substrate by migrating Physarum microamoebae. In parallel, we develop a mathematical model of a motile cell which includes forces from the viscous cytosol, a poro-elastic, contractile cytoskeleton and adhesive interactions with the substrate. Our experiments show that flow and traction stress exhibit back-to-front-directed waves with a distinct phase difference. The model demonstrates that the direction and speed of locomotion are determined by this coordination between contraction, flow and adhesion. Using the model, we identify forms of coordination that generate model predictions consistent with experiments. We demonstrate that this coordination produces near optimal migration speed and is insensitive to heterogeneity in substrate adhesiveness. While it is generally thought that amoeboid motility is robust to changes in extracellular geometry and the nature of extracellular adhesion, our results demonstrate that coordination of adhesive forces is essential to producing robust migration.

Both the basal body and the microtubule-based axoneme it nucleates have evolutionarily conserved subdomains crucial for cilium biogenesis, function and maintenance. Here, we focus on two conspicuous but underappreciated regions of these structures that make membrane connections. One is the basal body distal end, which includes transition fibres of largely undefined composition that link to the base of the ciliary membrane. Transition fibres seem to serve as docking sites for intraflagellar transport particles, which move proteins within the ciliary compartment and are required for cilium biogenesis and sustained function. The other is the proximal-most region of the axoneme, termed the transition zone, which is characterized by Y-shaped linkers that span from the axoneme to the ciliary necklace on the membrane surface. The transition zone comprises a growing number of ciliopathy proteins that function as modular components of a ciliary gate. This gate, which forms early during ciliogenesis, might function in part by regulating intraflagellar transport. Together with a recently described septin ring diffusion barrier at the ciliary base, the transition fibres and transition zone deserve attention for their varied roles in forming functional ciliary compartments.

The calcifying siphonalean green alga, Halimeda macroloba is abundant on coral reefs and is important in the production of calcium carbonate sediments. The process by which new green segments are formed over-night is revealed here for the first time.

The calcifying siphonalean green alga, Halimeda macroloba is abundant on coral reefs and is important in the production of calcium carbonate sediments. The process by which new green segments are formed over-night is revealed here for the first time.

The calcifying siphonalean green alga, Halimeda macroloba is abundant on coral reefs and is important in the production of calcium carbonate sediments. The process by which new green segments are formed over-night is revealed here for the first time.

The calcifying siphonalean green alga, Halimeda macroloba is abundant on coral reefs and is important in the production of calcium carbonate sediments. The process by which new green segments are formed over-night is revealed here for the first time.

The calcifying siphonalean green alga, Halimeda macroloba is abundant on coral reefs and is important in the production of calcium carbonate sediments. The process by which new green segments are formed over-night is revealed here for the first time.

Mitochondria are dynamic organelles in cells. The control of mitochondrial motility by signaling mechanisms and the significance of rapid changes in motility remains elusive. In cardiac myoblasts, mitochondria were observed close to the microtubular array and displayed both short- and long-range movements along microtubules. By clamping cytoplasmic [Ca2+] ([Ca2+]c) at various levels, mitochondrial motility was found to be regulated by Ca2+ in the physiological range. Maximal movement was obtained at resting [Ca2+]c with complete arrest at 1-2 microM. Movement was fully recovered by returning to resting [Ca2+]c, and inhibition could be repeated with no apparent desensitization. The inositol 1,4,5-trisphosphate- or ryanodine receptor-mediated [Ca2+]c signal also induced a decrease in mitochondrial motility. This decrease followed the spatial and temporal pattern of the [Ca2+]c signal. Diminished mitochondrial motility in the region of the [Ca2+]c rise promotes recruitment of mitochondria to enhance local Ca2+ buffering and energy supply. This mechanism may provide a novel homeostatic circuit in calcium signaling.

Mitochondria are dynamic organelles in cells. The control of mitochondrial motility by signaling mechanisms and the significance of rapid changes in motility remains elusive. In cardiac myoblasts, mitochondria were observed close to the microtubular array and displayed both short- and long-range movements along microtubules. By clamping cytoplasmic [Ca2+] ([Ca2+]c) at various levels, mitochondrial motility was found to be regulated by Ca2+ in the physiological range. Maximal movement was obtained at resting [Ca2+]c with complete arrest at 1-2 microM. Movement was fully recovered by returning to resting [Ca2+]c, and inhibition could be repeated with no apparent desensitization. The inositol 1,4,5-trisphosphate- or ryanodine receptor-mediated [Ca2+]c signal also induced a decrease in mitochondrial motility. This decrease followed the spatial and temporal pattern of the [Ca2+]c signal. Diminished mitochondrial motility in the region of the [Ca2+]c rise promotes recruitment of mitochondria to enhance local Ca2+ buffering and energy supply. This mechanism may provide a novel homeostatic circuit in calcium signaling.

In plant cells, myosin is believed to be the molecular motor responsible for actin-based motility processes such as cytoplasmic streaming and directed vesicle transport. In an effort to characterize plant myosin, a cDNA encoding a myosin heavy chain was isolated from Arabidopsis thaliana. The predicted product of the MYA1 gene is 173 kDa and is structurally similar to the class V myosins. It is composed of the highly-conserved NH2-terminal "head" domain, a putative calmodulin-binding "neck" domain an alpha-helical coiled-coil domain, and a COOH-terminal domain. Northern blot analysis shows that the Arabidopsis MYA1 gene is expressed in all the major plant tissues (flower, leaf, root, and stem). We suggest that the MYA1 myosin may be involved in a general intracellular transport process in plant cells.

Radiofrequency radiation (RFR) of 0.5 Hz - 2.5 GHz, at field strengths on the order of 10 v/cm, has been shown to interact with isolate Nitella internodal cells in two separate and frequency dependent manners. At frequencies up to ca. 10 KHz, RFR acts to directly excite the cell membrane, eliciting an action potential and halting cytoplasmic streaming. There is no reason to believe the RF induced action potentials and accompanying streaming cessations occur through a mechanism different from that which results from conventionally applied excitatory stimuli. Microscopic observations of the streaming in cells exposed to RFR gave no evidence of direct action by the radiation upon the mechanism which generates the motive force. Streaming alterations, however, did indirectly indicate the existence of a field induced disruption of the calcium ion release which normally occurs upon membrane excitation. On the bais of these observations and previously reported theoretical models of RF interactions with biological systems, a molecular mechanism for the electrically mediated release of calcium ion is presented.

Behavior of the centriolar duplexes during early mitosis was investigated. In general, both duplexes are capable of migrating about the cell as a unit with little change in their center to center spacing prior to separation to form the spindle poles. This duplex separation may occur at any point within the mid-prophase-prometaphase period. If it is delayed to prometaphase, transitory monopolar spindles were observed.

Behavior of the centriolar duplexes during early mitosis was investigated. In general, both duplexes are capable of migrating about the cell as a unit with little change in their center to center spacing prior to separation to form the spindle poles. This duplex separation may occur at any point within the mid-prophase-prometaphase period. If it is delayed to prometaphase, transitory monopolar spindles were observed.

The axonal migration of nucleic acids and their precursors has been studied following injection of radioactive uridine or thymidine into a single eye of newhatched chicks. After monocular injection of tritiated uridine, a progressive increase was observed in the specific activity of RNA of the optic lobe contralateral to the injected eye. This accumulation was first apparent 18 hours after injection and increased for at least 6 days. There was also a lesser accumulation of trichloracetic acid soluble radioactivity in this contralateral lobe relative to the ipsilateral lobe. RNA was prepared from morphological fractions of optic lobes after differential centrifugation. Radioactive analysis of these RNA fractions suggested that mitochondrial, ribosomal and transfer RNA all migrated axonally to a considerable degree. At neither 4 hours or 4 days after monocular injection of tritiated thymidine, was there any excess of radioactivity in DNA of lobes contralateral to the injected eye relative to DNA of ipsilateral lobes. Thus there was no evidence for the flow of DNA along the axon. The data suggest that a wide variety of RNA species synthesised in the nerve cell body migrate distally and appear at nerve endings. © 1971 Springer-Verlag.

The axonal migration of nucleic acids and their precursors has been studied following injection of radioactive uridine or thymidine into a single eye of newhatched chicks. After monocular injection of tritiated uridine, a progressive increase was observed in the specific activity of RNA of the optic lobe contralateral to the injected eye. This accumulation was first apparent 18 hours after injection and increased for at least 6 days. There was also a lesser accumulation of trichloracetic acid soluble radioactivity in this contralateral lobe relative to the ipsilateral lobe. RNA was prepared from morphological fractions of optic lobes after differential centrifugation. Radioactive analysis of these RNA fractions suggested that mitochondrial, ribosomal and transfer RNA all migrated axonally to a considerable degree. At neither 4 hours or 4 days after monocular injection of tritiated thymidine, was there any excess of radioactivity in DNA of lobes contralateral to the injected eye relative to DNA of ipsilateral lobes. Thus there was no evidence for the flow of DNA along the axon. The data suggest that a wide variety of RNA species synthesised in the nerve cell body migrate distally and appear at nerve endings. © 1971 Springer-Verlag.