Ephrin reverse signaling in axon guidance and synaptogenesis

Abstract

Axon-cell and axon-dendrite contact is a highly regulated process necessary for the formation of precise neural circuits and a functional neural network. Eph/ephrin interacting molecules on the membranes of axon nerve terminals and target dendrites act as bidirectional ligands/receptors to transduce signals into both the Eph-expressing and ephrin-expressing cells to regulate cytoskeletal dynamics. In particular, recent evidence indicates that ephrin reverse signal transduction events are important in controlling both axonal and dendritic elaborations of neurons in the developing nervous system. Here we review how ephrin reverse signals are transduced into neurons to control maturation of axonal pre-synaptic and dendritic post-synaptic structures.

1. Introduction

As the brain develops each neuron typically elaborates multiple neurite extensions that eventually differentiate into a single axon and multiple dendrites. Neural circuits form as each neuron wires together through both long-range axon elongation and pathfinding events that followed by more local short-range axon-dendrite contact with the neurons that it needs to target and form synapses with. During long-range pathfinding, axon growth cones extend and are guided to the appropriate target areas by various cues layed out in the environment, either as soluble, extracellular matrix associated, or cell membrane-bound ligand molecules [1]. In contrast, dendrites branch and arborize near the cell body to form small specific targeting structures and dendritic spines for axonal docking [2]. The connection and interaction of axon and dendrites give rise to structure modifications in both axon nerve terminals and dendritic protrusions to initiate the formation of specialized adhesive junctions, synapses. The processes of axon guidance and synaptogenesis are highly regulated during development and accumulating evidence show that a large number of cell surface receptors, cell adhesion molecules, and intracellular cytoskeletal regulators are implicated in the structure modifications that facilitate the formation and maturation of synapses [1, 3, 4].

The Eph family receptor tyrosine kinases and their membrane-archored ephrin ligands are one of the largest groups of membrane bound interacting proteins. Eph receptors are divided into two separate subfamilies, termed EphA and EphB. Ephrins are attached to the cell membrane either through a glycosylphosphatidylinositol (GPI) linkage (A-subclass ephrins) or by a hydrophobic transmembrane domain (B-subclass ephrins) that is followed by a short C-terminal intracellular segment [5]. Overall, EphA receptors bind in an overlapping fashion to the A-subclass ephrins and EphB receptors can promiscuously bind to B-subclass ephrins, although some cross-talk has been demonstrated [5–7]. The membrane association of both Ephs and ephrins brings about their unique feature of transducing signals bidirectionally into Eph expressing cells (forward signaling) and the ephrin expressing cells (reverse signaling) upon cell-cell contact (Fig. 1). The bidirectional signals mediated by Eph-ephrin interactions play diverse roles in a vast array of developmental processes including axon guidance [8]. In this review, we will summarize work that specifically focuses on the roles of ephrin reverse signaling in the outgrowth of axons and dendrites, and later in spine maturation and synapse formation. The roles of Eph receptor forward signaling in neural development and synaptic function have been reviewed elsewhere [8–12].

Figure 1.

Bidirectional signals mediated by Eph/ephrin in both presynaptic terminals and postsynaptic dendrites. Eph receptors are divided into two separate subfamilies, termed EphA and EphB. Their cognate ligands ephrins are attached to the cell membrane either through GPI linkage (ephrin-A) or by a hydrophobic transmembrane domain and intracellular segment (ephrin-B) which involves tyrosine (Y) phosphorylation residues and PDZ binding motif (V indicates valine) in the C-terminus. Upon axon-dendrites or axon-cell contact Eph and ephrin molecules become clustered and interact at the plasma membrane. Forward signals mediated by EphA and EphB receptors are transduced into both Eph expressed axon terminals and dendrites. Likewise, reverse signaling mediated by ephrin-A or ephrin-B are also transduced into both sides of the nerve junction.

2. Ephrin-A reverse signaling in axon guidance

The ephrin-A proteins are localized in lipid rafts, a specific microdomain of the plasma membrane due to the GPI linker and are entirely extracellular. The lipid raft provides a platform which allows ephrin-A to be constitutively associated and transduce reverse signals by interacting with other similarly localized molecules.

2.1 Ephrin-A associated binding molecules on the cell membrane

In addition to the ability of ephirn-As to interact with the EphA group of receptors [13, 14] and in some cases EphB2 [7], this subclass of ephrins also interact in cis with other co-expressed membrane-associated molecules and these interactions may help in the transduction of reverse signals. The first ephrin-A cis-acting associated molecules identified were the integrin class of cell adhesion molecules. Integrins are transmembrane molecules with many subtypes that mediate adhesion through interactions with the extracellular matrix. One subtype β1-integrin was found to interact with ephrin-A5 in the lipid raft and help mediate reverse signaling driven cell adhesion upon engagement with EphA receptor and is sufficient to sustain neurite outgrowth in retinal ganglion neurons [15]. The transduction of ephrin-A reverse signals also requires the Src-family kinase Fyn and Rho family small GTPases and leads to tyrosine phosphorylation of raft membrane proteins P80 and P120, which eventually brings about cytoskeletal rearrangement [16, 17]. Interestingly, activated Fyn is able to increase the surface amount of sphingomyelin clusters and hence inhibits the trafficking of ephrin-As from endosomes to plasma membrane, which serves as a negative feedback loop for ephrin-A signaling [18]. Besides integrins, neurotrophin receptors TrkB and p75NTR [19–22], and the ADAM10 membrane metalloprotease [23] have also been found to associate with ephrin-As on the plasma membrane. The association of ephrin-As with these membrane-associated proteins suggests a functional role for ephrin-A reverse signals in axon elongation and targeting. These ideas are described in the next section below.

2.2 Ephrin-A reverse signaling mediates axon pathfinding

Although numerous ephrin-A associated molecules have been identified, the underlying mechanisms for axon guidance in vivo are still elusive. In nervous system, ephrin-A mediated reverse signaling is implicated in the pathfinding of vomeronasal axons [24], spiny stellate cells of neocortex [25], spinal motor axons [26], olfactory receptor neuron axons [27], and retinal ganglion cell (RGC) axons [28].

In the chick and mouse retinotectal/retinocollicular topographic mapping, each RGC axon typically grows into the rostral tectum/superior colliculus (SC), overshoots its intended target area, and substantially retracts its processes to reach the appropriate rostral-caudal termination zone. Ephrin-As are well characterized tectal/SC-expressed repellant ligands for EphA forward signaling in rostral-caudal topographic mapping [29]. However, in contrast to the roles as ligands, ephrin-As have also been found to be expressed in an increasing temporal-to-nasal gradient in the retina and are capable of transducing reverse signals to guide axon projections into more caudal termination zones of the SC where its interacting receptor EphA7, acting as the repellent ligand, is expressed in a decreasing rostral-to-caudal gradient [28].

How do ephrin-As tranduce reverse signals to control axon motility? While the cell-based studies of Davy et al [15, 16] indicate cell adhesion is the response, the analysis of rostral-caudal mapping suggest that reverse signals mediate axon repulsion. Perhaps it’s a mix, involving an initial integrin adhesion response, which is consistent with the Eph and ephrin initially forming high affinity protein-protein interactions upon cell-cell or axon-cell contact. However, the biology of axon guidance rostral-caudal mapping indicates the ultimate outcome of ephrin-A reverse signaling is a retraction response. Some insight on this question has been obtained by studies of the other ephrin-A interacting proteins.

In RGC axons, ephrin-A molecules interact in cis with TrkB, a receptor for brain-derived neurotrophic factor (BDNF), and augments BDNF-promoted retinal axon branching, which is attributable to activation of the PI-3 kinase/Akt pathway [19, 21]. The ephrinA-evoked increase in branching is terminated by EphA-Fc in the stripe assay. This indicates that ephrin-A facilitates TrkB mediated axon growth initially but then brakes down the function upon ephrin-A:EphA interaction. How does this switch in axon growth occur? Further study showed that RGC axons can be also modulated in another way. Ephrin-A in RGC axons also interacts with p75NTR, another BDNF receptor, and EphA7/ephrin-A complex activated p75NTR to mediate axon repulsion [19, 22]. Axons lacking ephrin-A or of knockdown ephrin-A not only lost the ability to avoid an EphA matrix in stripe assay experiments, but also render RGC axons insensitive to BDNF [19]. These studies suggest an essential role of ephrin-A mediated reverse signaling in controlling RGC axon branching and provide a mechanism whereby ephrin-A mediated reverse signaling controls the switch from branch-promoting actively to axon repulsion/retraction via binding with different receptors, TrkB or p75NTR. Consistently, the RGC axon branching is controlled by the ligands for these receptors, BDNF and its precursor (proBDNF). BDNF promotes retina axon branching while proBDNF reduces it. The antagonistic actions of these ligands are attributable to the different ephrin-A/EphA complex formation with either BDNF/TrkB or proBDNF/p75NTR [20]. Therefore, ephrin-A reverse signals are transduced into two distinct pathways to execute antagonistic effects in the SC and the local ratio of the activities of BDNF/TrkB versus proBDNF/P75NTR mediated signals plays critical roles for appropriate development of the retinotopic map (Fig. 2).

Figure 2.

Distinct ephrin reverse signaling in presynaptic structure. Upon encountering Eph molecules expressed on the surface of target cells, ephrin-As interact with neurotrophin receptor TrkB and P75 NTR to regulate axon growth/attraction and retraction/pruning. Ephrin-A molecules also recruit ADAM10 to cleave the attachment of ephrin-As to terminate the signals. In contrast to ephrin-A molecules, ephrin-B molecules become clustered and tyrosine phosphorylated upon axon-cell contact. This results in Grb4 binding to ephrin-B3 via its SH2 domain and recruitment of both Dock180 and PAK via its second SH3 domain to increase Rac-GTP levels and downstream of active Rac for axon retraction/pruning. In addition, the PDZ binding motif in the C-terminus of ephrin-B is able to bind syntenin to induce presynaptic development.

Axon guidance mediated by Eph/ephrin binding initiated upon axon-cell or axon-dendrites contact represents an initial adhesive event due to the high affinity ephrin-A:EphA binding, but ultimately turns to a repulsive consequence. This raises the question how an initial tight Eph/ephrin physical binding becomes separated? Biochemical evidence shows that ephrin-A2 forms a stable complex in cis with ADAM10, a protease responsible for many membrane protein shedding processes, and Eph receptor binding triggered ephrin-A2 cleavage by this protease. The cleavage terminates EphA: ephrinA binding and is essential for ephrin-A reverse signaling mediated axon withdrawal [30]. Interestingly, another ephrin-A subtype, ephrin-A5, can also be cleaved in trans by ADAM10 expressed in EphA expressing cells [23]. Therefore, ephrin-A cleavase and specific proteolysis upon EphA binding, whether with ADAM10 presented in cis or in trans, plays a key role in terminating ephrin-A reverse signals for axon withdrawal.

Taken together, ephrin-A reverse signals in axons are transduced via directly interacting with different receptors on the cell membrane to control axon behavior from initial growth/branching activity to final repulsion/retraction events.

3. Ephrin-B reverse signaling in axon guidance and presynsptic development

In contrast to ephrin-As, the three different ephrin-Bs have an extracellular domain, a transmembrane domain and a cytoplasmic domain which enables these molecules to interact not only with membrane exposed proteins but also with intracellular proteins, which allow them to transduce distinct reverse signals into the cells they are expressed on.

3.1 Ephrin-B associated molecules on cell membrane

Ephrin-Bs have been demonstrated to interact in cis with numerous membrane expressed proteins such as fibroblast growth factor (FGF) receptor [31], adhesion molecules integrins [32], claudins [33], and connexin [34]. These molecules interacting with ephrin-Bs are thought to play roles in cell migration and cell-cell adhesion. For instance, FGF modulates ephrinB1 signaling to regulate the positioning of retinal progenitor cells within the definitive eye field [35, 36]. This association mediates critical cell movement and neural development in the Xenopus embryo (also see the review by Ira Daar). Further, ephrin-B cytoplasmic segment is able to interact with numerous intracellular proteins which allow it to transduce distinct reverse signals to control axon guidance and presynaptic development, which is discussed in detail below.

3.2 Ephrin-B serves as a receptor to mediate axon pathfinding

The critical roles of ephrin-B intracellular domain mediated reverse signaling in axon pathfinding was first found for midline crossing of anterior commissure axons in the mouse brain [37, 38]. Analysis of phenotypes associated with an EphB2 protein-null mutation and an intracellular truncated mutation lacking the conserved tyrosine kinase domain and C-terminal tail were compared to demonstrate EphB2 acts not as a receptor in axons, but rather as a ligand expressed in the ventral forebrain to guide the cortical axons that form the posterior tract of the anterior commissure across the midline of the brain [37]. These cortical anterior commissure axons were found to express ephrin-B molecules and using a similar strategy to delete the ephrin-B2 cytoplasmic domain, it was shown that reverse signaling mediated by ephrin-B2 acting as the receptor is key for formation of this commissure that connects the two sides of the cortex [38].

In the retina, ephrin-B expression is in a high dorsal low ventral gradient which is opposite to the high ventral to low dorsal gradient of EphBs. Analysis of EphB2/EphB3 double null and EphB2 truncated mutants indicate again that an EphB receptor kinase independent mechanism play essential roles in retina axon path finding [39]. Together with in vitro studies in which EphB receptors extracellular domains was found to induce growth cone collapse of axons from the dorsal retina, indicating a role of ephrin-B mediated reverse signaling in RGC axon guidance [40].

It is well known that RGC axons arising along the dorsal-ventral axis of the retina are distributed across the lateral-medial axis of the tectum/SC [41–43]. In RGC axon-SC topographic mapping, the EphB2 null phenotype was recapitulated in the EphB2 C-terminal truncation and indicates that forward signaling plays a dominant role for dorsal-ventral topographic mapping [41]. Interestingly, a recent study showed that while ephrin-B2 is expressed in a high dorsal-low ventral gradient in the embryonic retina, the ventral expression becomes more pronounced during postnatal stages, pointing towards a likely role for reverse signaling mediated by this molecule in both dorsal and ventral RGC axon retinocollicular mapping. By using various ephrin-B2 mutants that target key regions of the cytoplasmic domain needed for reverse signaling, it was found that both dorsal RGC axon termination zone formation, as predicted by its robust dorsal expression pattern, and ventral-temporal RGC axons were disrupted (Thacker et al, 2011, submitted). Combined the analysis of Eph/ephrin mutants, it is indicated both EphB forward and ephrin-B2 reverse signaling are key elements required for correct RGC axon-SC topographic mapping. Consistently, recent in vivo evidence showed that elevation of ephrin-B1 reverse signaling in RGC axons of the developing Xenopus retinotectal system promoted branch stability and increase the number of stabilized axon arbors but not substantially altered the overall growth of axon arbors [44], suggesting that ephrin-B reverse signaling has also a role in presynaptic maturation of visual connections.

Besides retinal axons, ephrin-B reverse signaling is involved in regulating diverse axon development. For instance, it is required for guidance of nucleus magnocellularis axons at the midline in the developing auditory brain [45], axon pruning of mossy fiber in hippocampus [46], formation of corpus callosum [47], axon pathfinding of from globular bushy cells in the ventral cochlear nucleus to medial nucleus of the trapezoid body on the controlateral side of the brainstem [48]. The requirement for ephrin reverse signals in these axon pathfinding events raises the need to obtain a clear understanding of the downstream molecular mechanisms utilized by the ephrin-Bs to transducer these signals into the axon growth cone.

3.3 Distinct pathways for ephrin-B reverse signal transduction in axons

How is ephrin-B reverse signaling transduced into axons? The signaling may involve at least two distinct molecular pathways governed by the ability of the ephrin-B intracellular domain to form protein-protein interactions with both SH2 and PDZ domain-containing intracellular proteins [49]. Following interactions with their cognate EphB receptors (and EphA4) and formation of circular tetramers and higher order clusters [50], the ephrin-B cytoplasmic tail becomes tyrosine phosphorylated by Src family kinase [51–54] and recruits the SH2/SH3 adaptor protein Grb4/Nckβ/Nck2 [52]. This pathway turns out to be critical in dentate granual cell axons, termed mossy fibers, where Grb4 plays an essential role for ephrin-B3 mediated axon pruning. In mossy fiber, Grb4 binds the tyrosine phosphorylated ephrin-B3 cytoplasmic tail through its the SH2 domain, and bridges ephrin-B3 with a number of intracellular proteins via its three SH3 domains, including the Rac guanine nucleotide exchange factor Dock180 and downstream effector Pak1, to bring about the retraction of growth cones [46].

PDZ binding motif is another important binding site within the intercellular segment of ephrin-B that is able to interact with a vast number of PDZ containing proteins to mediate ephrin-B reverse signaling. PDZ binding was found to be critical for certain axon development such as the formation of corpus callosum [47]. Although the downstream molecules responsible for propagating this signal in vivo has yet to be clearly elucidated, PDZ-RGS3 expression and localization has been shown within axons of corpus collosum and was suggested as one of the potential candidates important for migration [47, 55]. In addition, another PDZ containing molecule syntenin-1 was also found to bind ephrin-Bs [56] and mediate presynaptic ephrin-B1 and ephrin-B2 reverse signaling in presynaptic development [57]. Therefore, different ephrin-B intracellular binding motifs are able to mediate distinct signals through various protein-protein interactions while in vivo evidence is still needed to verify their roles in the axon development (Fig. 2).

Similar to A-subclass ephrins, ephrin-B reverse signals are initiated upon axon-cell contact and ephrin/Eph interaction which bring about attractive effects in many cases of development [58] but give rise to a repulsive response eventually that lead to axon withdrawal. This process involves termination mechanisms through which the ephrin-B expressing axon growth cones separate from cells they contact that express EphB receptors. However, in contrast to that of ephrin-As becoming cleaved by ADAM10, there are two other pathways so far that have been indicated in termination of ephrin-B:EphB interactions. One is through protein cleavage mediated by presenilin (PS) 1/ γ-secretase. Ephrin-B1 and ephrin-B2 are substrates of PS1, which is expressed in neurons and is associated with many cases of Alzheimer’s disease. Eph:ephrin binding triggers the cleavage of ephrin-Bs on the cell surface and stop ephrin-B mediated downstream signal transduction [59, 60]. The other pathway to terminate ephrin-B reverse signals is through an endocytosis mechanism upon Eph-ephrin mediated cell-cell contact. In the process, EphB-Ephrin-B complex can be internalized into either ephrin or Eph expressing cells and it is mediated by clathrin and lead to activation of Rac small GTPase [61–63], which can regulate cytoskeleton dynamics of plasma membranes at the axon terminal and lead to the collapse of growth cones.

Therefore, ephrin-B mediates diverse reverse signaling in axons through both extracellular and intercellular segments which bind other membrane-localized molecules and cytoplasmic proteins, respectively, to bring about repulsion/retraction of the growth cones from the contacting cells. Notably, ephrin-B molecules may continue to be expressed as axonal terminals further mature and develop into presynaptic terminals that form synapses with postsynaptic dendrites, in which the trans-synaptic Eph-ephrin-B reverse signals also play important roles in synaptic neurotransmission by mediating presynaptic release of neurotransmitter [64].

4. Postsynaptic roles of ephrin-A and ephrin-B reverse signaling

While the axon terminals mature and become presynapses, the dendrites on the postsynaptic side also develop coordinately to match the process of circuit formation. Ephrin-B molecules have also found to be expressed on the dendrites and postsynaptic membranes and interact with presynaptic EphB receptors, which serve as ligands in axon terminals, to stimulate reverse signals into the postsynases. This scenario of Eph-ephrin interaction will give rise to new mechanisms to be explored regarding our further understanding of ephrin reverse signaling in developing neurons.

In contrast to axon guidance and pathfinding, the development of dendrites, the postsynaptic structure, undergoes multiple steps of initial neurite outgrowth, branching and arborization, spine elaboration and maturation, and synapse formation. The properties of adhesive-repulsive switching and signaling mediated by Eph-ephrin interactions and signaling make them ideal to modulate the connections as synapses form. Although the presynaptic roles of ephrin reverse signaling has been extensively studied in axon development as discussed above, the understanding of postsynaptic ephrins and their specific roles in this unique cellular compartment of the neuron remain relatively limited. The evidence deciphering ephrin-mediated reverse signaling in postsynaptic structures are only now emerging.

4.1 Ephrin-A reverse signaling at postsynapses

Little is known regarding possible roles for ephrin-A mediated reverse signaling in the postsynapses. However, integrins, as mentioned above, are thought to participate in formation of dendritic spines and maturation of excitatory synapses [65, 66] although it is not known if this involves ephrin-A interactions. In addition, there is evidence that in hippocampal neurons ephrin-A interacts with TrkB receptor to control synaptogenesis [21]. Since these molecules are expressed in both pre- and post- synaptic structures, it is possible that postsynaptic development shares similar molecular mechanisms as that in axons. Further studies in vivo will be necessary to clarify the potential postsynaptic roles of ephrin-A mediated reverse signals.

4.2 Ephrin-B reverse signaling for dendritic morphogenesis

Dendrite development is initiated with neurite growth, branching and arborization before wiring to the presynaptic nerve terminals. As a mediator for forward signaling, the EphB2 receptor has been shown to be recruited in dendrites and play a critical role for establishment and maintenance of dendritic arbors [67]. Ephrin-B molecules are also expressed in dendrites and postsynapses as well. For example, among the three ephrin-B proteins, ephrin-B2 and ephrin-B3 are expressed specifically in CA1 pyramidal neurons in the hippocampus which form synapses with nerve terminals of Schaffer collateral, the axons from CA3 pyramidal neurons [68]. However, potential receptor like roles for ephrin-Bs in dendrite development remain poorly understood.

Similar to the reverse signaling in axons as mentioned above, ephrin-B is also able to transduce multiple signals into dendrites via distinct protein-protein interaction via its conserved intracellular binding motifs. Particularly in the CA1 neurons, ephrin-Bs become phosphorylated and can associate with SH2/SH3 adaptor protein Grb4 [69] or directly bind with PDZ containing proteins (Xu et al, 2011 submitted), to regulate dendrite morphogenesis. Recent work provides in vivo data revealed that ephrin-B3 is critical for postnatal dendrite development (Xu et al, 2011, submitted). The analysis of the dendritic branching associated with ephrin-B3 intracellular point mutations that disrupts different protein binding motifs revealed that both tyrosine phosphorylation/SH2 protein binding and PDZ protein binding are required for long-scale dendrite pruning (Xu et al, 2011, submitted). Therefore, the branching and arborization of the dendrites are regulated through multiple ephrin-B mediated reverse signaling pathways which work together to sculpt dendritic morphology (Fig. 3).

Figure 3.

Distinct ephrin-mediated reverse signaling in dendrites. Ephrin-A molecules are able to interact with TrkB receptors in neurons and promote synapse formation. Ephrin-B molecules transduce reverse signals to mediate both dendrite pruning and spinogenesis. Exuberant dendritic processes are pruned by ephrin-B reverse signal transduction through interacting with Grb4 via SH2 binding motif and syntenin via PDZ binding motif, respectively. Spine maturation and synapse formation, on the other hand, require the participation of Grb4/GIT1 pathway, Pick1/syntenin/GRIP PDZ binding interactions, and ERK1/2 cascade.

4.3 Ephrin-B reverse signaling for postsynapse formation

Synapses are the key functional structures that connect axon terminals with postsynaptic cells and dendritic spines and are formed as the neurons develop and mature. Since Ephrin-Bs are expressed specifically in CA1 pyramidal neurons, the essential role of ephrin-B reverse signals for synaptic plasticity has been demonstrated at hippocampal CA3-CA1 synapses of adult brain [68]. The question remains if ephrin-B molecules play a role in synaptogenesis? Further studies in cultured neurons were performed to investigate the role of ephrin-Bs in dentrites and postsynaptic structure [69–72]. Although ephrin-B1 is only weakly expressed in the hippocampus, it has been reported to transducer reverse signals into postsynaptic neurons by recruiting Grb4 and the G protein couple receptor interacting protein (GIT) 1 to regulate spine morphogenesis and synapse formation [69]. Ephrin-B3, however, is highly expressed in the hippocampus and was shown to play an essential role for shaft synapse formation through its PDZ binding motif interacting with glutamate receptor-interacting protein (GRIP) 1 [71]. Recently, ephrin-B reverse signaling was also found to regulate synapse formation in cultured cortical neurons by inhibition of ERK/MAP kinase signaling [57].

In addition to in vitro studies, recent data on dendritic spines development obtained in vivo using various ephrin-B3 mutant mice deciphered how the intracellular components of reverse signaling mediate CA1 postsynaptic development (Xu et al, 2011, submitted). Interestingly, only PDZ binding appears to be required for spine maturation along the dendrites. Further studies in cultured neurons reveal that two PDZ containing proteins, syntenin1 and Pick1, interact with ephrin-B3 and mediate the reverse signals to modulate spine maturation and synapse formation. Taken together, ephrin-B reverse signaling is also involved in the development of postsynaptic structures. Protein-protein interaction with different ephrin-B intracellular binding motifs mediates the reverse signals and controls the dendrite arborization, spine maturation, and synapse formation (Fig. 3).

5. Conclusion and Perspective

In summary, ephrin-A and ephrin-B molecules transduce distinct reverse signals upon interacting with their cognate Eph partners acing as ligands. These reverse signals play critical roles for sculpting developing axons and dendrites during early development. In axons, ephrin-As locate within lipid rafts and interact with neurotrophin receptors TrkB and P75NTR to control axon pathfinding and topographic maping, while ephrin-Bs are able to recruit various downstream cytoskeletal regulators to mediate axon guidance and presynaptic development (Fig. 2). In contrast to the axon development, dendrites are also sculpted via ephrin-B reverse signals and distinct downstream protein-protein interactions to promote maturation of postsynaptic structures (Fig. 3).

While Eph/ephrin bidirectional signaling is well known to participate in neuronal development, still little is known how these signals are utilized to contribute to the processes of sculpting a sophisticated but highly organized neural circuitry that helps form the neural network and maintain its plasticity. In fact, accumulating evidence reveals that in the maturing and adult brain ephrin-Bs participate in synaptic plasticity by either modulating presynatic release [64, 73] or by interacting with NMDA and AMPA receptors at postsynapses [68, 70, 74, 75] to help regulate LTP/LTD. In the developing Xenopus optic tectum, ephrin-B participates in regulating visual experience-dependent and developmental plasticity of repetitive fields, indicating an essential role of Eph/ephrin in the synaptic plasticity of neural circuits upon environmental stimuli [76]. The study of Eph/ephrin bidirectional signaling upon developmental axon-dendrite contact or plasticity inducing external stimuli in the mature brain would be a great starting point to investigate the neural wiring and circuit formation in vivo [77–79] and this would lead to better understanding the intrinsic mechanisms of brain function and behavior such as sensory signal input, learning and memory, anxiety, and neurodegeneration in disease brain.

Highlights.

• >Ephrin-A and ephrin-B mediate reverse signals into both axons and dendrites of developing neurons
• >Ephrin-As interact with axonal membrane-located receptors to control axon behavior
• >Ephrin-Bs interact with various intercellular proteins to regulate axon guidance and presynaptic development
• >Distinct ephrin-B reverse signals are involved in dendrite branching, spine maturation and synapse formation