Intraretinal projection of retinal ganglion cell axons as a model system for studying axon navigation

Abstract

The initial step of retinal ganglion cell (RGC) axon pathfinding involves directed growth of RGC axons toward the center of the retina, the optic disc, a process termed “intraretinal guidance”. Due to the accessibility of the system, and with various embryological, molecular and genetic approaches, significant progress has been made in recent years toward understanding the mechanisms involved in the precise guidance of the RGC axons. As axons are extending from RGCs located throughout the retina, a multitude of factors expressed along with the differentiation wave are important for the guidance of the RGC axons. To ensure that the RGC axons are oriented correctly, restricted to the optic fiber layer (OFL) of the retina, and exit the eye properly, different sets of positive and negative factors cooperate in the process. Fasciculation mediated by a number of cell adhesion molecules (CAMs) and modulation of axonal response to guidance factors provide additional mechanisms to ensure proper guidance of the RGC axons. The intraretinal axon guidance thus serves as an excellent model system for studying how different signals are regulated, modulated and integrated for guiding a large number of axons in three-dimensional space.

Introduction

Proper wiring of the nervous system is of fundamental importance for the function of the nervous systems and remains to be one of the fascinating and challenging areas in biology (Tessier-Lavigne and Goodman, 1996). Due to its relative simplicity and accessibility, the vertebrate neural retina has been a favorite model system for both neurobiologists and developmental biologists. During embryonic development, bilateral evagination of the neural tube gives rise to the optic vesicle (Fig. 1A). Subsequent invagination of the optic vesicle results in the formation of a double-layered cup and the inner layer of the optic cup becomes the prospective neural retina (Fig. 1B). The process of invagination also forms a groove at the ventral side of the retina connecting with the optic stalk, called the “optic fissure”. Later, when the retinal tissues at both sides of the optic fissure expand and fuse, the first retinal ganglion cell (RGC) axons exit the eye through the optic fissure. In addition to the RGC axons, mesenchymal cells also migrate through the optic fissure into the eye cup to form the retinal artery. Surrounding the emerging blood vessels and the exiting axons, the “optic disc” is a circular structure that forms at the center of retina.

Within the neural retina, multipotent progenitor cells differentiate into six major types of neurons and one type of glial cell (Dyer and Cepko, 2001, Livesey and Cepko, 2001, Yang, 2004). The retinal ganglion cells are the first cell type to differentiate, and the differentiated ganglion cell soma reside in the ganglion cell layer (GCL) at the vitreal side of the retina. However, later in development, the GCL is not exclusively occupied by the ganglion cell soma. Approximately 50% of the cells in the GCL are displaced amacrine cells that synapse with the ganglion cells (Drager and Olsen, 1981, Jeon et al., 1998). Shortly after the ganglion cells differentiate, the axons extend toward the vitreal surface and there, they turn and extend to form the optic fiber layer (OFL), along with the neuroepithelial endfoot processes (Fig. 1D) (Halfter et al., 1983, Holt, 1989, Prada et al., 1981). The optic fiber layer is thus very close to the vitreal surface, just below the inner limiting membrane, a thin basal lamina.

The retinal ganglion cells are the sole output neuron in the retina, relaying visual information by sending long axons to project to the brain. The long journey of the RGC axon pathfinding can be divided into the following segments. First, the axons from the RGCs distributed throughout the cup-shaped retina have to project toward the optic fissure/disc where they exit the eye. This precisely regulated process is called “intraretinal axon targeting” (Goldberg and Coulombre, 1972). Next, after exiting the eye, the RGC axons arrive at the optic chiasm, where they interact with the scaffold of local cells in the midline and with axons from the contralateral eye. In mouse and human, a subset of the axons project to the contralateral side of the brain whereas the rest stay at the same side as the retina. In chick and fish, all RGC axons appear to cross to the contralateral side (O'Leary et al., 1983, Polyak, 1957). Finally, the axons reach the part of the brain, the optic tectum in non-mammalian vertebrates or the superior colliculus (SC) in mammals. Particular challenges that the RGC axons face are to establish specific connections in the tectum (or SC) to form a “topographic map” so that the neighboring axons in the retina project onto the adjacent sites in the tectum (or SC). Remarkable progress has been made in recent years in understanding the molecular mechanisms involved in RGC axon pathfinding at the optic chiasm (Mann and Holt, 2001, Mason and Erskine, 2000) and the establishment of the retinotectal map (Flanagan, 2006, Lemke and Reber, 2005, McLaughlin and O'Leary, 2005).

In this review article, I will focus on the first segment of the RGC axon navigation, the intraretinal axon guidance. There are approximately 50,000 RGCs in mouse, 1.5 million in human and 2.4 million in chick. The axons from the RGCs differentiated at different times and distributed throughout the retinal cup have to be guided to the optic disc. Intraretinal axon targeting thus serves as an excellent model system for studying how axons receive and integrate guidance information to navigate in three-dimensional space. This system can also be used for regeneration study in the hope of curing nerve damages occurred in injury or neurodegenerative diseases. A large number of molecules have been characterized to affect RGC axon growth in culture. However, the role of these molecules in intraretinal axon guidance requires additional consideration of proper spatiotemporal expression patterns and in vivo gain-of-function and loss-of-function experiments. I will thus place special emphasis on in vivo studies in the context of intraretinal axon pathfinding. Due to the limit of space, it is not possible to include all the relevant literature, and the readers are referred to several excellent reviews for detailed discussion of some of the earlier work (Oster et al., 2004, Stuermer and Bastmeyer, 2000, Thanos and Mey, 2001).