Carl Franck

Cornell University, Ithaca, New York, United States

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Publications (17)15.39 Total impact

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    ABSTRACT: Unicellular eukaryotic amoebae Dictyostelium discoideum are generally believed to grow in their vegetative state as single cells until starvation, when their collective aspect emerges and they differentiate to form a multicellular slime mold. While major efforts continue to be aimed at their starvation-induced social aspect, our understanding of population dynamics and cell cycle in the vegetative growth phase has remained incomplete. Here we show that cell populations grown on a substrate spontaneously synchronize their cell cycles within several hours. These collective population-wide cell cycle oscillations span millimeter length scales and can be completely suppressed by washing away putative cell-secreted signals, implying signaling by means of a diffusible growth factor or mitogen. These observations give strong evidence for collective proliferation behavior in the vegetative state.
    Physical Biology 04/2014; 11(3):036001. · 2.62 Impact Factor
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    ABSTRACT: Living cells depend upon the detection of chemical signals for their existence. Eukaryotic cells can sense a concentration difference as low as a few per cent across their bodies. This process was previously suggested to be limited by the receptor-ligand binding fluctuations. Here, we first determine the chemotaxis response of Dictyostelium cells to static folic acid gradients and show that they can significantly exceed this sensitivity, responding to gradients as shallow as 0.2% across the cell body. Second, using a previously developed information theory framework, we compare the total information gained about the gradient (based on the cell response) to its upper limit: the information gained at the receptor-ligand binding step. We find that the model originally applied to cAMP sensing fails as demonstrated by the violation of the data processing inequality, i.e. the total information exceeds the information at the receptor-ligand binding step. We propose an extended model with multiple known receptor types and with cells allowed to perform several independent measurements of receptor occupancy. This does not violate the data processing inequality and implies the receptor-ligand binding noise dominates both for low- and high-chemoattractant concentrations. We also speculate that the interplay between exploration and exploitation is used as a strategy for accurate sensing of otherwise unmeasurable levels of a chemoattractant.
    Journal of The Royal Society Interface 08/2013; 10(88):20130606. · 4.91 Impact Factor
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    ABSTRACT: Physicists can look to dilute suspensions of apparently solitary cells in suspension for elegant realizations of multicellular behavior. In contrast to our earlier work (Phys. Rev. E v. 77, 041905 (2008)) with the amoeba Dictyostelium discoideum we are discovering that the vital intercellular communications responsible for the well-known but poorly understood slow to fast transition in a growing culture as a function of time might be due to the passage of chemical messages between transient cell clusters or throughout the entire system as opposed to binary collisions. In considering the observed variation in proliferation rates we have been surprised to discover that for best growth cultures are much more dependent on incubator geometry than previously suspected.
    03/2013;
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    ABSTRACT: Unlike inanimate condensed matter, living cells depend upon the detection of chemical signals for their existence. First, we experimentally determined the chemotaxis response of eukaryotic Dictyostelium cells to static folic acid gradients and show that they can respond to gradients as shallow as 0.2% across the cell body. Second, using Shannon's information theory, we showed that the information cells receive about the gradient exceeds the theoretically predicted information at the receptor-ligand binding step, resulting in the violation of the data processing inequality. Finally, we analyzed how eukaryotic cells can affect the gradient signals by secreting enzymes that degrade the signal. We analyzed this effect with a focus on a well described Dictyostelium cAMP chemotaxis system where cAMP signals are affected by an extracellular cAMP phosphodiesterase (PDE) and its inhibitor (PDI). Using a reaction-diffusion model of this set of interactions in the extracellular space, we show that cells can effectively sense much steeper chemical gradients than naively expected (up to a factor of 12). We also found that the rough estimates of experimental PDE and PDI secretion rates are close to the optimal values for gradient sensing as predicted by our model.
    03/2013;
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    ABSTRACT: Eukaryotic cell flattening is valuable for improving microscopic observations, ranging from bright field to total internal reflection fluorescence microscopy. In this talk, we will discuss traditional overlay techniques, and more modern, microfluidic based flattening, which provides a greater level of control. We demonstrate these techniques on the social amoebae Dictyostelium discoideum, comparing the advantages and disadvantages of each method.
    03/2010;
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    ABSTRACT: In recent work we explored the notion that the transition between slow and fast growth, the lag-log transition, with increasing density seen in shaken cell culture represents a collective effect. (Phys. Rev. E 77, 041905 (2008)). We reported preliminary observations in which the lag phase was apparently missing. Here, we present significantly more measurements than in our original work as well as increased sensitivity at low densities. We confirm that instances of nearly exponential (``log'') growth do in fact appear, but more frequently, we find evidence of lagging. The degree of lagging fluctuates significantly from run to run, in contrast to our earlier observations and theory, but in all cases exponential growth is established with increasing density once the range of 10^4 to 10^5 cells/ml is reached. We present evidence against two natural explanations for these fluctuations: 1) a mixture of strains which have different growth phenotypes or 2) a single strain variation due to an epigenetic switch which can be set to the low growth state by subjecting cells to high density environments. The appearance of such growth variations has considerable practical significance and suggests that there is an additional dynamical variable besides density in play.
    03/2010;
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    ABSTRACT: Eukaryotic cell flattening is valuable for improving microscopic observations, ranging from bright field (BF) to total internal reflection fluorescence (TIRF) microscopy. Fundamental processes, such as mitosis and in vivo actin polymerization, have been investigated using these techniques. Here, we review the well known agar overlayer protocol and the oil overlay method. In addition, we present more elaborate microfluidics-based techniques that provide us with a greater level of control. We demonstrate these techniques on the social amoebae Dictyostelium discoideum, comparing the advantages and disadvantages of each method.PACS Codes: 87.64.-t, 47.61.-k, 87.80.Ek.
    PMC Biophysics 01/2010; 3(1):9.
  • Carl Franck, Kayvon Daie
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    ABSTRACT: The starvation response of the eukaryotic microbial system Dictyostelium discoideum has continued to provide opportunities to explore the transition from solitary to collective life. Specifically, one observes a change of behavior from random to synchronized cellular motion reflecting successful long-ranged chemical signaling that leads to aggregation. In the typical experimental universe life goes on upon a flat substrate underneath an ocean of liquid media through which these chemical signals pass. In our observations of starvation development we have uniquely exploited the possibilities afforded by varying the depth of this signaling channel over an interesting range: from essentially infinitely thick (mm's of depth) to an extremely thin wetting layer (below 1 micron). We also examine the development system over a wide range of surface density: from almost a full monolayer to a few percent areal coverage. Our key observation is a striking reduction of the time from the beginning of starvation to the onset of synchronized movement when we reduce the aqueous overlayer thickness to the thinnest values. We provide an interpretation for our observations by combining an exact solution to the diffusive transport problem with a rough dynamical theory for multiagent synchronization. This work was supported by the NIH (P01 GM078586).
    03/2009;
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    ABSTRACT: The population dynamics of microbial life in sheared liquid suspension affords opportunities to explore the ways in which cells encourage each other to proliferate. Such elegant systems continue to inspire us to develop and test simple theories for cooperative behavior (e.g. Phys. Rev. E v. 77, p. 041905 (2008)) in living matter. We report on new insight afforded by the observation of the amoebae Dictyostelium discoideum of the effect on population growth of the introduction of adhesive contacts of cells with each other as well as solid substrates. Through a hydrodynamic scaling argument we find that that mechanical triggers provided by intercellular collisions are more important than collisions with container walls in encouraging growth. Finally, we confirm the discovery of a strain that lacks growth regulation due to density sensing. This work was supported by the NIH (P01 GM078586).
    01/2009;
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    ABSTRACT: In cell culture, when cells are inoculated into fresh media, there can be a period of slow (or lag phase) growth followed by a transition to exponential growth. This period of slow growth is usually attributed to the cells' adaptation to a new environment. However, we argue that, based on observations of shaken suspension culture of Dictyostelium discoideum, a model single-cell eukaryote, this transition is due to a density effect. Attempts to demonstrate the existence of implicit cell signaling via long-range diffusible messengers (i.e., soluble growth factors) through cell-medium separation and microfluidic flow perturbation experiments produced negative results. This, in turn, led to the development of a signaling model based on direct cell-to-cell contacts. Employing a scaling argument for the collision rate due to fluid shear, we reasonably estimate the crossover density for the transition into the exponential phase and fit the observed growth kinetics.
    Physical Review E 04/2008; 77(4 Pt 1):041905. · 2.31 Impact Factor
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    ABSTRACT: Although intercellular communication is frequently viewed as involving the transport of small molecules through an intracellular fluid medium, biologists have proposed chemical signaling with chemical specificity due to chemical recognition through direct contacts. Considering the collective computation behind the decision of a cell to divide when it senses the presence of a sufficient number of like neighbors, we offer a model for the transition from slow to exponential growth in shaken suspension cell culture of the model eukaryote, Dictyostelium discoideum. Besides exploring an elegantly simple example of multicellular life, this discussion might well prove useful in considering the limits of cell culture on small spatial scales as required for contemporary massively parallel biotechnology.
    03/2008;
  • Albert Bae, Wui Ip, Carl Franck
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    ABSTRACT: In eukaryotic cell culture, it is routinely recommended to keep the cells above a minimum cell density to maintain vigorous growth. We are investigating the basis for this prescription by viewing cell growth as a social behavior facilitated by cell-cell communication. Employing Dictyostelium discoideum, we find good evidence for a slow-fast transition in the cell growth rate vs. density in well mixed, 25 ml, cell cultures. We also use low height microfluidic chambers (four orders of magnitude smaller in volume) to find similar behavior even though the system is not well mixed and the cells are confined to substrates. A preliminary measurement at a flow rate that should strongly perturb cell-cell communication by means of diffusing signal molecules suggests that cell communication essential for growth is not accomplished by such means but possibly via direct contacts.
    03/2007;
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    ABSTRACT: When confronted by starvation, collections of the amoeba Dictyostelium discoideum seek to aggregate in order to form genome-preserving stalk and spore structures. We have been interested in the means by which individual cells unite for this purpose. It has long been recognized that communication by means of diffusion of small molecules affords one such strategy: periodic chemical wave signaling can direct individual cells to an aggregation site. By employing thin layer substrates that presumably alter the propagation characteristics of such waves, we have shifted the colonial aggregation strategies to modes that rely on adhesive interactions for initial stages of multicellular assembly. Besides relentless aggregation of individual cells into large scale streams, these substrates reveal remarkable structures composed of only a few cells which we call ``squads'' that search for each other in order to achieve sufficient aggregation mass in sparse populations.
    03/2007;
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    ABSTRACT: The chemotactic response of Dictyostelium discoideum cells to stationary, linear gradients of cyclic adenosine 3',5'-monophosphate (cAMP) was studied using microfluidic devices. In shallow gradients of less than 10(-3) nM/microm, the cells showed no directional response and exhibited a constant basal motility. In steeper gradients, cells moved up the gradient on average. The chemotactic speed and the motility increased with increasing steepness up to a plateau at around 10(-1) nM/microm. In very steep gradients, above 10 nM/microm, the cells lost directionality and the motility returned to the sub-threshold level. In the regime of optimal response the difference in receptor occupancy at the front and back of the cell is estimated to be only about 100 molecules.
    European Journal of Cell Biology 10/2006; 85(9-10):981-9. · 3.21 Impact Factor
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    ABSTRACT: A new method for stirring thin liquid films has been developed and demonstrated to increase the sensitivity of immunofluorescence staining of polytene chromosomes. This liquid-on-liquid mixing (LOLM) technique uses a stirrer fluid, immiscible with the thin film, to transmit shear at the liquid-liquid interface. Here, we stir mineral oil layered over an aqueous thin film of antibody solution, which stains transcription apparatuses on chromosomes previously fixed to a glass slide. The quality of staining was assessed at varying antibody concentrations and incubation or stirring times. Our data indicate that the LOLM technique overcomes the diffusion barrier associated with traditional slide-based biological assays.
    Journal of Biochemical and Biophysical Methods 08/2005; 64(1):59-68. · 2.33 Impact Factor
  • Richard C. Yeh, Carl Franck
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    ABSTRACT: Diffusion limits the transport of biopolymer reactants in the thin aqueous films found in many biomedical assays. This bottleneck prevents the reactions, such as DNA hybridization on microarrays, from reaching chemical equilibrium, and limits the overall speed, sensitivity, and reproducibility of the assays. We have been solving this problem by using a well stirred immiscible bulk confining liquid to transmit effective large-scale mixing flow patterns into the thin film, a method we call Liquid-on-Liquid Mixing (LOLM). The LOLM technique is shown to improve the speed and sensitivity of immunofluorescence staining of polytene chromosomes. This project is supported by the New York State Office of Science, Technology, and Academic Research.
    03/2004;
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    ABSTRACT: The study of the starvation response of the social amoeba system Dictyostelium discoideum has long been regarded as an important opportunity to learn about the transition to multicellular life for eukaryotic org anisms. Here we focus on two critical developmental steps: the first indication that a co mmunications network between formerly independent cells has been established as evidenced by synchronized periodic shape changes and the first stages of aggregation a s seen by the development of the streams of migrating cells. In contrast to earlier work which has stressed large scale structures, to be interpreted with continuum theori es, we focus our attention on the smallest scales of interest for multicellular effec ts. In order to better understand the means by which these developmental steps come about we study their timing from the start of starvation as a function of a hitherto und erappreciated experimental variable: the thickness of the aqueous layer phase through which the chemical messages diffuse between the cells while the cells themselves crawl about on a substrate. We also examine the importance of sparseness in such microbial life by examining these transitions as a function of the substrate's coverage by cells, agai n with particular attention to the smallest scale multicellular features. In contrast to earlier theoretical approaches that rely