Elsevier

Brain Research Bulletin

Volume 79, Issue 5, 30 June 2009, Pages 227-247
Brain Research Bulletin

Review
Key roles of Ephs and ephrins in retinotectal topographic map formation

https://doi.org/10.1016/j.brainresbull.2009.03.008Get rights and content

Abstract

Cellular and molecular mechanisms involved in the development of topographic ordered connections in the central nervous system (CNS) constitute a key issue in neurobiology because neural connectivities are the base of the CNS normal function. We discuss the roles of the Eph/ephrin system in the establishment of retinotopic projections onto the tectum/colliculus, the most detailed studied model of topographic mapping. The expression patterns of Ephs and ephrins in opposing gradients both in the retina and the tectum/colliculus, label the local addresses on the target and give specific sensitivities to growth cones according to their topographic origin in the retina. We postulate that the highest levels of these gradients could signal both the entry as well as the limiting boundaries of the target. Since Ephs and ephrins are membrane-bound molecules, they may function as both receptors and ligands producing repulsive or attractant responses according to their microenvironment and play central roles in a variety of developmental events such as axon guidance, synapse formation and remodeling. Due to different experimental approaches and the inherent species-specific differences, some results appear contradictory and should be reanalyzed. Nevertheless, these studies about the roles of the Eph/ephrin system in retinotectal/collicular mapping support general principles in order to understand CNS development and could be useful to design regeneration therapies.

Introduction

The functions of the nervous system depend upon precisely organized neuronal connections. The dominant model to study the development of topographic ordered neuronal connections is the projection from the retina to its major midbrain target namely the optic tectum of frogs, fishes and chicks or its mammalian homologue, the superior colliculus. These connections are formed during embryonic development and maintained throughout life. Many axonal projections within the brain establish an orderly arrangement in their target field. These fields are organized in such way that the spatial arrangement of the soma is reflected in the order of their axonal terminations. Hence, neighboring cells project to neighboring parts of the target forming a continuous biological construction that could be illustrated as a map. The spatial order of axonal terminations of retinal ganglion cells (RGCs) in the tectum/colliculus is organized in two orthogonally oriented axes [135] (Fig. 1). Nasal RGCs project to the caudal tectum/colliculus and the temporal ones project to the rostral tectum/colliculus. In the other axis, the dorsal RGCs contact the ventral (lateral) tectum/colliculus and the ventral RGCs contact the dorsal (medial) tectum/colliculus.

Optic fibers invade the tectum/colliculus from the rostral pole, follow the tectal developmental gradient axis toward the caudal pole and establish synaptic connections in the retinorecipient layers [15], [195], [196], [215]. The mechanisms responsible for the establishment of these topographic connections have been investigated for several years and have now begun to be understood at the molecular level [8], [35], [49], [61], [224], [225].

The topographic mapping mainly depends upon two successive developomental events: (1) the specification of topographic cellular identities in the retina and the tectum/colliculus, which in time, establishes (2) the pattern of expression of superficial cue labels and receptors that participate in axon guidance during establishment of topographic ordered connections [4], [64], [83], [118], [130], [133], [135], [142], [148], [155], [171], [192], [198], [199], [215], [232].

Besides, interactions among axons play a central role allowing the younger axons to contact and follow the pioneer axons to reach the target [97], [164], [175], [194], [215] and mediating interaxonal competition.

Projecting axons grow over long distances before reaching their final targets. Along the way, growth cones located at the leading edges of axons detect and respond to guidance cues. These molecular cues may be classified as long-range and short-range signals [214]. The former group includes soluble molecules such as semaphorins, slits, netrins, and neurotrophins [102], [147] whereas the short-range signals include matrix-associated molecules such as laminin [24], [25], [81], [108], [134] or proteoglycans [11], [164], [215] and cell-associated molecules such as cell adhesion molecules (CAMs) [16], [46], [221], [236], cadherins [104], [212] and ephrins [135], [166]. The growth cones function as sensory–motor units that respond with morphological changes to these cues. Thus, the collapse or spreading of growth cones can be elicited by repulsive or attractant signals, respectively.

Many individual guidance cues, however, can function both as repellents or attractants in different conditions. This guidance feature depends on several factors, such as the intracellular status of the growth cone, which includes the differential expression of receptor complexes and the cross-talk among intracellular signaling cascades. Growing axons sense and respond to a myriad of guidance cues and the mechanisms involved in the integration of this information are now being unraveled [52], [92], [102], [147], [151], [158], [220].

Positional information is deciphered by receptors expressed in the growth cones. These receptors activate signaling pathways that include Rho family GTPases (Rho, Rac1, Cdc42), Ras family proteins, heterotrimeric G proteins [37], [38], [52], [102], [150], [151], [220], Src family kinases [115], mitogen-activated protein kinases (MAPK) [53], [80], focal adhesion kinase [9], [19], [143], [151], among others. These signaling pathways regulate the function of effector molecules that sculpt the axon morphology by remodeling the cytoskeleton, the plasma membrane, the extracellular matrix and the adhesion.

Finally, when growth cones enter the target area, they respond to molecular cues that guide axons to ramify forming termination zones in specific areas where branches search for their target neurons. Depending on: (a) the bidirectional communication between axon branches and tectal neurons and (b) the interaction among growing axons, a regulated process of adhesion induces the synapse formation. Depending on the presence of partner adhesion molecules, synchronic spontaneous and triggered activity, some synapses are stabilized whereas others are eliminated during the process of connections refinement.

This review focuses on basic principles of topographic mapping by guidance molecules expressed in gradients. Specifically, we will discuss the roles of Eph tyrosine kinase receptors and their membrane-bound ligands, the ephrins [54], in the formation of retinotectal/collicular projections.

Section snippets

Target arrival: interstitial versus terminal branching

Optic fibers invade the tectum/colliculus from the rostral pole and follow the tectal developmental gradient axis toward the caudal pole. These axons form termination zones (TZs) by producing branches in an appropriate area along the rostrocaudal/dorsoventral plane of the tectum/colliculus. Then, they invade the retinorecipient layers where they establish synaptic connections [15], [122], [195], [196], [215]. The formation of these TZs is a multistep process in chicks and rodents, whereas it

How can the RGCs axons establish the correct location of their TZs? Shall we go back to sperrificity?

Data on visual system formation have grown in complexity during last years opening further questions to be discussed. The complete understanding of intracellular signaling is a matter of time but it does not assure by itself the understanding of the entire phenomenon. Thus, an excessive focus on the molecular aspects of topographic connections could be blinding. Cell–cell communication requires more than protein–protein interaction, as topographic connections require more than cell–cell

Lets put ephrins in the map

Graded expression of Eph receptors and their ligands, the ephrins, both in the retina and the optic tectum/superior colliculus has suggested that these molecules could provide positional information that guides the topographic movement of growth cones in the visual system and other regions of the developing nervous system [26], [35], [49], [61], [141], [168]. The analysis of the roles of Eph/ephrin system in mapping retinal projections onto rostrocaudal and ventrodorsal axes of the

EphAs/ephrin-As

EphAs and ephrin-As have been shown to be involved in defining the topographic retinotectal/collicular connections along the rostrocaudal axis whereas EphBs and ephrin-Bs have been shown to be implicated in the dorsoventral axis [61], [135], [136], [168], [215]. A prominent role for EphAs and ephrin-As in establishing topographic ordered connections appears to be to inhibit interstitial branching of RGCs axons posterior to their correct TZs in chicks and rodents [187], [234]. EphBs and

EphAs/ephrin-As

Ephrin-A2 and ephrin-A5 located in the caudal tectum/colliculus are growth cone repellents [35], [49], [58], [65], [146], [153] and interstitial branching inhibitors [191], [234] that preferentially affect temporal RGCs axons by activating their EphA (A3 and A4 in chicks, A4 and A5 in mice) receptors [12], [59] (Fig. 1).

Monschau et al. [146] showed that ephrin-A2 is repellent for temporal axons, whereas ephrin-A5 repels both types of retinal axons, with temporal axons being more sensitive than

Could the highest levels of the gradients signal both the entry as well as the boundaries of the target cellular domain?

It is possible to appreciate that different expression patterns of Ephs and ephrins along the retinotectal/collicular pathway have different actions on axon guidance. Hence, borders expressing high levels of ephrins have a repulsive effect that prevent the axons to invade an inappropriate domain, such as ephrin-Bs expressed in optic chiasm prevent EphB-bearing RGCs axons from crossing the midline of the brain [152], [226]. Conversely, gradients of ephrins and Ephs signal the local addresses

Is it possible for Ephs and ephrins to interact in cis?

In chicks, RGCs express both Ephs and ephrins [26], [94], [99], [141]. Using immunocytochemistry, Hornberger et al. [99] showed a patch-like appearance of ephrin-A5 and ephrin-A2 expression on chicken RGCs axons, possibly due to clustering into lipid rafts [41], [42], [101]. Accordingly, Campbell et al. [18] described distinct membrane compartimentalization and signaling of ephrin-A5 and ephrin-B1 transfected in murine fibroblasts and tissues in vitro and in vivo[18]. Marquardt et al. [129]

Other positional labels play complementary and redundant functions in retinotectal/collicular mapping

Although the Ephs and ephrins constitute the best characterized molecular system in mapping, other candidates have been identified (Fig. 8).

Works in knock-out mice have shown that in the abscense of ephrin-As (A2/A5/A3), a rough retinocollicular map still forms [58], [65], [172], implicating the existence of other potential guidance cues [14]. Ephrin-A6 (in chicks), the repulsive guidance molecule (RGM) and engrailed 2 (En-2) may play this role. En-2, which is expressed in an increasing

Eph/ephrin system also participates in synapse formation and plasticity

Although synapse formation and plasticity is an issue that is not discussed in works about mapping, this process is the final step in targeting [193] and Eph/ephrin system has been also involved in this developmental event.

Is the Eph/ephrin system a barrier to or is it necessary for regeneration?

Following damage to the CNS in avians and mammals, different cell types respond in different ways. Neurons try to regenerate their connections, and largely fail. Astrocytes and microglial cells proliferate, migrate and become hypertrophic and oligodendrocytes attempt to remyelinate. Expression of Ephs and ephrins in each of these situations may affect the response of these cells to the damage. Given the role of Ephs and ephrins in axon guidance during development, a common view is that these

Concluding remarks

In order to discuss the roles of the Eph/ephrin system in the axon guidance during the establishment of topographic ordered connections in the CNS, we analyzed the development of the retinotectal/collicular system because this is the main experimental model to study this issue.

Comprehension of cellular and molecular mechanisms involved in the establishment of topographic ordered connections in the CNS constitutes a necessary starting point to understand the normal function of the CNS and to

Conflict of interest

The authors state that they have no conflict of interest.

Acknowledgements

This work was supported by grants from University of Buenos Aires (UBACYT M096 and M439) and CONICET (PIP 6003, 6176 and 112-200801-03135).

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