Patch coalescence as a mechanism for eukaryotic directional sensing
Abstract Book of the XXIII IUPAP International Conference on Statistical Physics, Genova, Italy, (9-13 July 2007)Abstract
The cells of multicellular organisms are endowed with a chemical compass of
amazing sensitivity, formed as a result of billion years of evolution 1.
Concentration differences of the order of a few percent in the extracellular
attractant chemicals from side to side are sufficient to induce a
chemical polarization of the membrane leading to cell migration towards the
signal source. This way, a sensible amplifier of slight gradients in the
distribution of chemicals in the surrounding environment is realized. Its
relevance is easily understood if one recognizes that no multicellular
organism could exist without the constituent cells being able to organize
themselves following chemical cues. The directional sensing process starts when a slight anisotropic component in the extracellular chemical signal induces the separation of the cell membrane into two sharply defined domains, populated by different phospholipid molecules and oriented along the signal anisotropy. It has been realized recently that this early polarization process is the result of a phase-separation instability in a well-characterized network of diffusion-controlled chemical reactions 2,3. We show that it is possible to give a universal description of this early simmetry breaking process,
articulated into subsequent stages of patch nucleation, coarsening and merging
into a single domain, following the lines of the Lifshitz-Slezov theory for the growth of clusters of a stabler phase in a metastable sea.
Our description explains the existence of two clearly
separated polarization regimes depending on the presence or absence of an
anisotropic component in the activation pattern produced by the extracellular
attractant factor, and the recent observation of a sensitivity threshold for the anisotropic component 4. In particular, we find that the polarization time
$t_\epsilon$ in the presence of an anisotropic extracellular signal depends
on the anisotropy degree $\epsilon$ through the power law $t_\epsilon
\epsilon^- 2$, and that in a cell of radius $R$ there should exist a
threshold value $\epsilon_th R^- 1$ for the smallest
detectable anisotropy. Our results do not depend on the microscopic details of the directional sensing network and are in agreement with existing experimental
data.
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1) A. Ridley, M. Schwartz, K. Burridge, R. Firtel, M. Ginsberg, G. Borisy, J.
Parsons, A. Horwitz, Science 302 (2003) 1704.\\
2) A. Gamba, A. de Candia, S. Di Talia, A. Coniglio, F. Bussolino, G. Serini,
Proc. Nat. Acad. Sci. U.S.A. 102 (2005) 16927.\\
3) A. de Candia, A. Gamba, F. Cavalli, A. Coniglio, S. Di Talia, F. Bussolino, G. Serini, Science's STKE 379 (2007) pl1.\\
4) L. Song, S.M. Nadkarni, H.U. Bodeker, C. Beta, A. Bae, C. Franck, W.J. Rappel, W.F. Loomis, E. Bodenschatz,
Eur. J. Cell Biol. 85 (2006) 981.