Mechanisms of Gradient Detection: A Comparison of Axon Pathfinding with Eukaryotic Cell Migration

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Abstract

The detection of gradients of chemotactic cues is a common task for migrating cells and outgrowing axons. Eukaryotic gradient detection employs a spatial mechanism, meaning that the external gradient has to be translated into an intracellular signaling gradient, which affects cell polarization and directional movement. The sensitivity of gradient detection is governed by signal amplification and adaptation mechanisms. Comparison of the major signal transduction pathways underlying gradient detection in three exemplary chemotaxing cell types, Dictyostelium, neutrophils, and fibroblasts and in neuronal growth cones, reveals conserved mechanisms such as localized PI3 kinase/PIP3 signaling and a common output, the regulation of the cytoskeleton by Rho GTPases. Local protein translation plays a role in directional movement of both fibroblasts and neuronal growth cones. Ca2+ signaling is prominently involved in growth cone gradient detection. The diversity of signaling between different cell types and its functional implications make sense in the biological context.

Introduction

Graded distributions of chemotropic factors (referred to as “gradients” herein) are essential guideposts in the development, regeneration, and function of multicellular organisms. They provide directional as well as positional information and steer migrating cells and neuronal growth cones. Detection of gradients allows Dictyostelium cells to aggregate and proceed in their life cycle, neutrophils to migrate to sites of infection, fibroblasts to invade and heal wounds, and neuronal growth cones to follow guidance cues and establish the complex connectivity of the nervous system. It should be noted, however, that directed movement can be established by cues other than graded distributions of guidance factors. Likewise, gradients can induce cellular responses different from directed movement. These topics will not be considered in this article.

Gradient detection by eukaryotic cells differs fundamentally from the one in bacteria. Because bacteria are too small to sense a concentration gradient along their cell length, they use a temporal gradient‐sensing mechanism. Dependent on temporal changes in the concentration of a chemoattractant or a chemorepellent, they regulate the frequency of random directional reorientation by changing the direction of flagellar rotation. This allows them to bias their overall direction of movement. Although highly efficient in detecting minimal concentration differences over a wide range by means of adaptation, a temporal gradient‐sensing mechanism can be only employed by a moving cell and does not guide cells on a straight path toward or away from a chemotactic factor (Baker 2005, Wadhams 2004).

A migrating eukaryotic cell, on the other hand, can sense an external concentration gradient along the cell length and translate it into an internal signaling gradient. The internal gradient leads to a morphological polarization and, in some cases, an asymmetry of sensitivity. Once polarized, the cell initiates directed movement via cytoskeletal rearrangements. Similarly, a neuronal growth cone responding to an attractive or repulsive gradient activates differing signaling events at the side facing the higher concentration (gradient near side) and the side facing the lower concentration (gradient far side), while performing a turning response. To detect small concentration differences and translate them efficiently into a correlated turnaround, the external gradient has to be amplified by means of signal transduction. Furthermore, a cell or a growth cone migrating in a gradient possibly has to adjust its sensitivity and adapt to detect concentration differences over a broad range of absolute concentrations.

In this chapter, we will exemplarily review these different aspects of gradient detection during chemotaxis in two amoeboid cell types (Dictyostelium and mammalian neutrophils) as well as in fibroblasts and compare them with gradient detection of neuronal growth cones. Necessarily, this focus on a few model systems excludes many other important cell types, which perform chemotaxis in response to gradients such as metastatic cancer cells (Condeelis et al., 2005), mesoderm cells during vertebrate gastrulation (Dormann and Weijer, 2006), germ line cells (Kunwar et al., 2006), or mammalian sperm (Eisenbach and Giojalas, 2006).

To give a representative and detailed picture of eukaryotic chemotaxis, Dictyostelium cells and mammalian neutrophils are particularly suited, because they have been extensively studied in this regard and can serve as an exemplary model for chemotaxis of other eukaryotic cell types (Dormann 2006, Williams 2006). Although evolutionary distant, Dictyostelium cells and neutrophils are morphologically similar and share common pathways for gradient detection (Charest and Firtel, 2006). Dictyostelium cells chemotax toward gradients of cAMP, meaning they move up a gradient of a single chemoattractant. Neutrophils are attracted by gradients of different chemotactic factors, which are released in the case of infection or inflammation. Both cell types detect external gradients of a chemoattractant with high sensitivity and perform a strong internal signal amplification by means of feedback loops comprising phosphatidylinositol‐3 kinase (PI3K) and its catalytic product, phosphatidylinositide 3,4,5‐tris‐phosphate (PIP3). Dictyostelium cells and neutrophils slightly differ, however, regarding the role of the cytoskeleton during gradient detection.

Compared to amoeboid cells, fibroblasts are larger and have a different, more complex cytoskeletal architecture resembling the one in neuronal growth cones. Fibroblasts can sense and migrate up an attractant gradient of platelet‐derived growth factor (PDGF) but are much slower than neutrophils. The intracellular signaling underlying gradient detection in fibroblasts shares key components with Dictyostelium and neutrophils. However, fibroblasts lack important feedback mechanisms and therefore have a low sensitivity and a strong dependence on the absolute PDGF concentration when navigating in attractant gradients (Schneider and Haugh, 2005). Interestingly, they employ local protein synthesis at the leading edge to promote directional growth (Shestakova et al., 2001), a principle that is also observed in growth cone guidance (Piper and Holt, 2004).

At first view, neuronal growth cone guidance differs substantially from cell migration. Growth cones advance and navigate relatively independently from the neuronal cell body. Morphologically, growth cones are already polarized and have an intrinsic bias between the axon shaft and the protruding peripheral domain. In the nervous system, growth cones face attractive as well as repulsive gradients and respond to different classes of guidance factors, which can substantially differ in their downstream signaling. Nonetheless, many general features of eukaryotic gradient detection are conserved between migrating cells and growth cones. It is the aim of this review to point out these features and establish common ground between two related fields of research.

Section snippets

Chemotaxis of Dictyostelium

Dictyostelium cells detect and migrate up gradients of cAMP. In a cAMP gradient, the cells adopt a strong internal signaling polarity, which arises from localized activities of phosphatidylinositol‐3 kinase (PI3K) and the PI3‐phosphatase PTEN as well as a corresponding internal gradient of phosphatidylinositide 3,4,5‐tris‐phosphate (PIP3) along the cell membrane. As a result, the cytoskeleton is rearranged, morphological polarization becomes apparent, and the cell starts to move in the

Signaling pathways

The signal transduction during eukaryotic gradient detection shares a number of conserved pathways (Table 1.2). Dictyostelium cells and neutrophils have been frequently compared in terms of signaling, and their mode of gradient detection has been explained based on common models (Charest 2006, Skupsky 2005, Van Haastert 2004). Fibroblast gradient detection is accomplished by similar signaling pathways, although it is much simpler as in amoeboid cells and restricted with respect to signal

Concluding Remarks

Chemotaxis is an important task for eukaryotic cells in different biological and functional contexts. Migrating cells as well as neuronal growth cones are specialized to detect gradients of chemotactic factors by a spatial gradient sensing mechanism. During spatial gradient sensing, the external gradient has to be translated into an internal signaling gradient across the length of the cell or the growth cone, respectively. This signaling asymmetry or internal polarization normally results in a

Acknowledgments

We thank F. Weth and C. Gebhardt for critical reading and suggestions on the manuscript. This work was supported by the DFG (grant 1034/14‐1 to M.B.). A. P. received a stipend from the German National Scholarship Foundation.

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