ReviewConservation and divergence of axon guidance mechanisms
Introduction
Axon guidance is a specialised form of cell movement in which only part of the cell, the growth cone, is motile (for a general review of axon guidance, see [1]). A neuronal growth cone expresses cell adhesion molecules (CAMs) that enable it to move by modulating its adhesion to the extracellular matrix and to other cells. In addition to adhesion, neuronal growth cones are guided by both attractive and repulsive extracellular cues. Such cues may be diffusible (e.g. netrins and semaphorins) or attached to the cell surface (e.g. ephrins), and they can promote growth cone collapse (i.e. repulsion) or stabilisation (i.e. attraction) depending on the specific receptors that the growth cone expresses and on the cellular responses that those receptors evoke. In vivo, a growth cone must integrate all these cues to guide the axon correctly.
Ten years ago, there was little expectation that studies of axon guidance in simple invertebrates such as Drosophila or C. elegans would provide insights into understanding more complex vertebrate nervous systems. However, it is now clear that, at a molecular level, many guidance mechanisms are conserved to a remarkable degree amongst these organisms.
The characterisation of axon guidance mechanisms in evolutionarily distant animals allows a comparative approach to understanding axon guidance. The powerful genetics of the model animals can be used to identify new interacting components of these mechanisms, with the expectation that such components will be conserved. It is currently thought that nematodes, insects, and vertebrates shared a common ancestor in the Precambrian — the ‘urbilaterian’ animal (see Figure 1) [2]. Any mechanism common to all three phyla will probably be present in all bilaterian animals, and must also have been in the ancestral animal. Here, we ask what mechanisms of axon guidance are conserved amongst the three ‘model phyla’, and what aspects may have diverged. Such a comparative approach can provide valuable insights into axon guidance mechanisms specific to a particular species, as well as illuminate our understanding of the evolution of nervous systems in all animals.
As the conservation of molecules is the most unambiguous guide to the conservation of mechanisms, our presentation will focus on particular ligands and receptors that stimulate, inhibit, and direct axonal growth. This review is not intended to be exhaustive and focuses on those molecular species for which there is good in vivo loss-of-function evidence for a role in axonal guidance; molecular species that have been implicated less directly in axon guidance are not described (see [1] for a further discussion). We start by discussing two bifunctional sets of secreted axon guidance molecules, the netrins and the more recently characterized Slit proteins and their receptors, before moving to two large families of repellent proteins, the ephrins and semaphorins. We close with a discussion of other mechanisms, including receptor protein tyrosine phosphatases.
Section snippets
Netrins and their receptors
Netrins are a small family of highly conserved guidance cues (molecular weight ∼70–80 kDa) (Figure 2), with one known member in C. elegans (UNC-6) and two in Drosophila (netrin-A and -B); in vertebrates, two members were originally identified in chick (netrin-1 and -2) (see [1] for a review). Orthologues of netrin-1 have been described in the mouse, human, frog and zebrafish (reviewed in [3]). Another netrin that does not appear to be the orthologue of either netrin-1 or netrin-2 has been
Contact repulsion: ephrins and Eph receptors
Ephrins and their receptors, the Eph receptor tyrosine kinases, mediate growth cone collapse and thus axon repulsion. Eph receptors comprise the largest subfamily of receptor tyrosine kinases in vertebrates, and can be divided into two subclasses (EphA and EphB) based on sequence and ligand specificity [45]. Their ligands, the ephrins, are attached to cell surfaces by glycosylphosphatidylinositol (GPI) anchors (ephrin-As) or transmembrane domains (ephrin-Bs) (Figure 2). In general, EphA
Semaphorins
Whereas studies of netrins, Slit proteins, and ephrins have all emphasized the remarkable molecular and functional conservation of these guidance cues in axon and cell migration, studies on the semaphorin family illustrate the extent to which guidance molecules can be conserved in a general sense while exhibiting strong evolutionary divergence at a detailed mechanistic level.
The first semaphorins were identified in grasshopper (Sema-1a/fasciclin IV) and chicken (Sema3a/collapsin-1/Sema III)
Modulation of axon adhesion: receptor protein tyrosine phosphatases
Receptor-like protein tyrosine phosphatases (RPTPs) form a large family of transmembrane proteins that contain intracellular tyrosine phosphatase catalytic domains. RPTPs with extracellular domains consisting only of Ig-like domains and FNIII repeats have been found in flies, worms, and vertebrates, and have been classified as type IIA (LAR-related) or type III RPTPs (see Figure 2). Some of these RPTPs are expressed in neurons and have been implicated in axon guidance on the basis of genetic
Some non-conserved mechanisms?
Some molecules implicated in axon guidance in one animal have not yet been shown to have conserved roles in other systems; this may reflect truly divergent mechanisms or may simply be because the relevant homologs have not yet been identified. For example, the vab-8 gene functions in many posteriorly directed axon outgrowth and cell migrations in C. elegans [97]. vab-8 encodes multiple intracellular kinesin-like proteins that appear to function cell autonomously in the migrating cell or axon
Closing comments
The past decade has seen the birth of a new comparative approach to mechanisms of axon guidance. Many cell signaling pathways involved in axon guidance are conserved between insects, nematodes, and vertebrates, and probably arose by divergent evolution from an ancestral metazoan. In general, vertebrates have greater numbers of each ligand or receptor, perhaps reflecting genome duplications in the vertebrate lineage [101]. Some mechanisms may be specific to vertebrates (e.g. transmembrane
Note added in proof
Two studies in press provide evidence that the neuropilin co-receptor is indeed a plexin family member 104, 105.
At the proof stage, it was discovered that an important reference [106•], extending the results reported in [19], had been inadvertently omitted from the text.
Acknowledgements
We thank Katja Brose for help in preparing Figure 2.
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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2017, NeuroToxicologyCitation Excerpt :Furthermore, research shows that such directional extension of the axons occurs ubiquitously in both vertebrates and invertebrates and is regulated by several axon guidance cues including netrins, semaphorins, slits, and ephrins (Charron et al., 2003; Dickson and Senti, 2002; Fujisawa, 2004; Katow, 2008; Keynes and Cook, 1995; Kidd et al., 1999; Kolodkin et al., 1992; Wu et al., 1999). Netrins were initially proposed as midline-derived axon guidance cues that direct axon migration towards the ventral midline during embryogenesis in all bilaterally symmetrical animals (Chisholm and Tessier-Lavigne, 1999; Dickson, 2002; Dickson and Keleman, 2002; Guthrie, 1997; Rajasekharan and Kennedy, 2009; Tessier-Lavigne and Goodman, 1996). Hpnetrin, a netrin homolog in the sea urchin (Hemicentrotus pulcherrimus), has been shown to mediate serotonergic axon guidance in this basal deuterostome (Katow, 2008).