Orientation of Cell Polarity by Chemical Gradients

Abstract

Accurate decoding of spatial chemical landscapes is critical for many cell functions. Eukaryotic cells decode local chemical gradients to orient growth or movement in productive directions. Recent work on yeast model systems, whose gradient sensing pathways display much less complexity than those in animal cells, has suggested new paradigms for how these very small cells successfully exploit information in noisy and dynamic pheromone gradients to identify their mates. Pheromone receptors regulate a polarity circuit centered on the conserved Rho-family GTPase, Cdc42. The polarity circuit contains both positive and negative feedback pathways, allowing spontaneous symmetry breaking and also polarity site disassembly and relocation. Cdc42 orients the actin cytoskeleton, leading to focused vesicle traffic that promotes movement of the polarity site and also reshapes the cortical distribution of receptors at the cell surface. In this article, we review the advances from work on yeasts and compare them with the excitable signaling pathways that have been revealed in chemotactic animal cells.

Keywords: Cdc42; chemotaxis; chemotropism.

Fig. 1.. Polarity and gradient sensing.

A) Cdc42-related proteins act as molecular switches, turned on by GEFs and off by GAPs. B) Polarity establishment involves switching from a homogeneous distribution of Cdc42 to a polarized distribution with concentrated active Cdc42 defining the cell’s front. C) Chemical signals (Ligand) are sensed by specific G-protein-coupled receptors (GPCR) at the membrane, which cause GTP-loading of Gα and its separation from G Gβγ. The active Gα-GTP and Gβγ go on to influence Cdc42 activation. Gα-GTP can be deactivated by regulators of G-protein signaling (RGS). D) External gradients (blue) influence the location of polarity establishment (top) and can cause relocation of the polarity site (bottom).

Fig. 2.. Models for polarity establishment.

A) Recruitment of GEF to the membrane by active Cdc42 provides positive feedback (green arrow). B) Simplified MCAS model captures key dynamics of polarization. The cytoplasmic (rapidly diffusing) substrate v is interconverted to the membrane-associated (slow diffusing) activator u. C) Symmetry breaking triggered by noise. D) When more than one initial polarity site forms, competition for substrate leads to a single site at steady state.

Fig. 3.. Ratiometric sensing improves gradient detection.

A) With uniformly distributed receptors, a gradient of pheromone (blue) is faithfully translated into a similar gradient of bound receptors, allowing accurate polarization (green). B) Unevenly distributed receptors could mislead a cell: despite higher pheromone concentration at left, there would be more bound receptors at right. Internal dots represent active (green) and inactive (grey) G proteins. C) Mechanism for ratiometric sensing: G protein activation is catalyzed by bound receptor, while inactivation is catalyzed by RGS protein that is recruited to the membrane by unbound receptor. Thus, net activation reflects the ratio of bound to unbound receptor. D) Ratiometric sensing compensates for uneven receptor density, allowing accurate gradient detection. E) Measurement of mean pheromone concentration (Y axis: bound receptors) is noisier in a system that uses ratiometric sensing (blue) than one that directly senses bound receptors (yellow). F) When cells with uniformly distributed receptors are exposed to a pheromone gradient (blue arrows), there is both a gradient of bound receptors (LR) and an inverse gradient of unbound receptors (R). Systems with ratiometric sensing exploit both gradients to generate steeper internal active G protein gradients (green dots) than those with non-ratiometric sensing. Internal arrows indicate resultant vectors from positions of all active G proteins. G) The magnitude of the resultant vector of active G protein is greater in systems with ratiometric sensing (blue) compared to non-ratiometric sensing (yellow), leading to reduced noise in directional sensing (variability in angle between resultant vector and external gradient).

Fig. 4.. Speed dating in fission yeast.

A) Mating occurs in crowded mixtures of cells of the two mating types (blue and yellow). B) Cells sequentially polarize and depolarize Cdc42 (green), emitting puffs of pheromones locally when polarized. Eventually, assembly of aligned polarity sites leads to sensing of high pheromone levels by receptors and G proteins concentrated at each polarity site, which stabilizes the sites to allow fusion.

Fig. 5.. Pheromone sensing in budding yeast.

A) Free Gβγ recruits Ste5 from the cytoplasm, leading to activation of a MAPK cascade. B) Free Gβγ recruits Far1 from the cytoplasm, leading to local activation of Cdc42 and polarization of actin by formins (Cdc42 effectors that nucleate actin polymerization). Actin cables then deliver secretory vesicles to the polarity site. C) When cells polarize Cdc42 in the wrong location, the polarity site relocates to align with the partner. In cells with high MAPK activity, relocation involves movement of the polarity sites until the sites become aligned and the cells mate. D) Actin cables that deliver vesicles follow the polarity site as it moves. E) Off-center vesicle delivery perturbs Cdc42 by adding membrane (dilution) and GAPs (inactivation) on one side of the polarity site. Positive feedback is then stronger on the opposite side, and the centroid of the Cdc42 cluster can move persistently away from the actin-mediated vesicle delivery side. F) Directional bias in polarity site movement imparted by pheromone gradient (blue arrow). 1: A polarity site (green: Cdc42 concentration profile) creates a local neighborhood enriched in receptors (yellow: active GPCR concentration profile). 2: A polarity site spontaneously moves to the left (down-gradient: top) or to the right right (upgradient: bottom). 3: As it moves, the polarity site deposits new receptors in its wake. If the site moves down-gradient (top), fewer of the new GPCRs will be activated by pheromone than if the site moves up-gradient (bottom). 4: The activated GPCR gradient is translated into a Cdc42 GEF gradient by the Far1 pathway, biasing movement to return the site towards its earlier position (ping-pong movement). The differences in GPCR activation due to the pheromone gradient lead to a difference in the magnitude of this effect, so that sites that moved down-gradient are strongly biased to return up-gradient (top), while those that moved up-gradient can remain close to the new position.

Fig. 6.. Parallels between gradient decoding in yeast and motile cells.

A) In yeasts, a self-organizing polarity circuit centered on Cdc42 assembles a polarity site via positive feedback, and makes it mobile via negative feedback (R, repressor). Pheromone sensing by GPCRs impacts Cdc42 location primarily by a single pathway that recruits GEF to sites with bound receptors. Cdc42 orients actin cables that deliver vesicles, which both perturb Cdc42 (enhancing mobility) and deliver new receptors (enabling local sensing of pheromone). B) In motile cells, complex excitable signaling and cytoskeletal circuits (F, self-reinforcing “front” factors; R, repressors of F that can create local “refractory” state) generate spontaneous locomotion. GPCR sensing displays adaptation and has been simplified as a Local Excitation/Global Inhibition (LEGI) incoherent feedforward loop (E, excitation factors; I, inhibition factors; RR, response regulators that integrate E and I). A memory module, also simplified as an incoherent feedforward loop (factors as in LEGI), endows cells with directional persistence. C) Generalized roles for positive feedback in generating a polarized front, and for negative feedback in destabilizing the position of the front, enabling relocation to track a gradient.