Axon guidance receptors: Endocytosis, trafficking and downstream signaling from endosomes

Highlights

• Axon guidance receptors traffic in and initiate downstream signaling from endosomes.

• Localization to specific endosomes recruits downstream proteins to regulate guidance responses.

• Endosomal localization is not pre-determined, rather receptors can re-route to trigger specific guidance response.

Abstract

During the development of the nervous system, axons extend through complex environments. Growth cones at the axon tip allow axons to find and innervate their appropriate targets and form functional synapses. Axon pathfinding requires axons to respond to guidance signals and these cues need to be detected by specialized receptors followed by intracellular signal integration and translation. Several downstream signaling pathways have been identified for axon guidance receptors and it has become evident that these pathways are often initiated from intracellular vesicles called endosomes. Endosomes allow receptors to traffic intracellularly, re-locating receptors from one cellular region to another. The localization of axon guidance receptors to endosomal compartments is crucial for their function, signaling output and expression levels. For example, active receptors within endosomes can recruit downstream proteins to the endosomal membrane and facilitate signaling. Also, endosomal trafficking can re-locate receptors back to the plasma membrane to allow re-activation or mediate downregulation of receptor signaling via degradation. Accumulating evidence suggests that axon guidance receptors do not follow a pre-set default trafficking route but may change their localization within endosomes. This re-routing appears to be spatially and temporally regulated, either by expression of adaptor proteins or co-receptors. These findings shed light on how signaling in axon guidance is regulated and diversified - a mechanism which explains how a limited set of guidance cues can help to establish billions of neuronal connections. In this review, we summarize and discuss our current knowledge of axon guidance receptor trafficking and provide directions for future research.

Introduction

The wiring of the nervous system during embryogenesis requires for neurons to extend their axons towards distant targets to generate synaptic connections and establish functional circuits. The growth cone, a sensory structure at the axon tip, navigates the axon through the surrounding environment by detecting so-called axon guidance proteins that are expressed along predetermined trajectories. Axon guidance proteins are detected by specialized receptors expressed at the growth cone surface and trigger intracellular signaling cascades that induce axon steering by regulating the growth cone cytoskeleton. Axon guidance proteins exist both as membrane-associated cues, acting at short range, or as secreted molecules with longer range effects. Further, they can elicit attractive or repulsive responses, i.e. direct the growth cone towards or away, respectively, from a cue-expressing structure (Kolodkin and Tessier-Lavigne, 2011). Five different families of canonical guidance proteins have been identified: Netrins, Slits, Repulsive Guidance Molecules, Semaphorins and ephrins. Netrin-1 proteins signal through deleted in colorectal cancer (DCC), Neogenin and UNC-5 receptors, while proteins from the roundabout (Robo) family act as Slit receptors. RGMs require Neogenin as a receptor, and Semaphorins bind and activate Neuropilin and Plexin receptors. Finally, ephrins signal through Eph receptors (Kolodkin and Pasterkamp, 2013; Pasterkamp and Kolodkin, 2013) (Fig. 1). In addition to these canonical axon guidance proteins additional protein families previously recognized for other functions have been implicated in axon guidance (Augsburger et al., 1999; Avilés et al., 2013; Hollis and Zou, 2012). Interestingly, the effects of axon guidance proteins are not restricted to axons, instead these cues can also act as guidance cues for a plethora of different (non)neuronal cell types, in addition to regulating processes such as cellular proliferation, differentiation and survival (Jongbloets and Pasterkamp, 2014; Klein, 2012; Sun et al., 2011; Ypsilanti et al., 2010). It is therefore not surprising that changes in the expression or function of axon guidance proteins underlie a variety of neurological conditions (Van Battum et al., 2015).

For neurons to respond to their environment, signals received by receptors located at the growth cone surface need to be transmitted and processed within the cell. Even though many receptors can signal from the plasma membrane, other receptors are internalized and start their signaling cascades intracellularly. In the latter situation, activated receptors are endocytosed and subsequently sorted and trafficked - a process in which receptors are re-located from one cellular region to another. To do so, these receptors are embedded within membrane structures called endosomes and transported actively. Rab-GTPases localize on the membrane of these endosomes which marks their identity but also regulates their activity (Burd and Cullen, 2014; Naslavsky and Caplan, 2018). This transport may be short distance, for example the immediate recycling back to the adjacent cell surface, or long distance, such as from distal regions of growth cones, axons or dendrites back to the soma. Moreover, receptor trafficking is not only important to re-locate receptors within neurons, but also provides the initial steps for the activation of downstream signaling (Burd and Cullen, 2014; Burk et al., 2017a; Harrington and Ginty, 2013; Mellman, 1996; Sorkin and Von Zastrow, 2009). In this review, we will summarize our current knowledge of axon guidance receptor trafficking, highlight which endosomal compartments are involved in the trafficking of specific receptors, and discuss how receptors are regulated as a result of endocytosis and trafficking.