Molecular determinants of retinal ganglion cell development, survival, and regeneration
Introduction
The retina is a part of the CNS that is particularly well accessible for quantitative analysis of both physiological processes and their regulation during development, and pathophysiological processes and experimental manipulation through adulthood. While containing a vast number of different subtypes of cells, the retina is constituted by seven main cell types that are becoming increasingly well characterized, both with respect to their molecular determinants, morphology, anatomical location and integration into functional circuits, neurotransmitters employed, and changes in pathological processes. For the main retinal cell types, timing of generation, maturation, and connectivity have been determined.
In this paper, we concentrate on one particular cell type in the retina, the retinal ganglion cells (RGCs). RGCs are the only projection neurons in the retina, extending their axons through the optic nerve (ON), the optic chiasm, and the optic tract (OT) into their midbrain targets, namely the superior colliculus (SC) and lateral geniculate nucleus (LGN) of mainly the contralateral side of the brain (Fig. 1).
The development of the eye and in particular the neural retina has been a focus of several recent reviews (Chow and Lang, 2001; Easter and Malicki, 2002). Apart from morphogenetic structuring, also the molecular determinants controlling retinal development (for review see Kumar, 2001), and the pathophysiology underlying retinal degeneration including RGC and photoreceptor degradation in genetic and stress-induced pathologies are becoming increasingly understood (Goldblum and Mittag, 2002; Rakoczy et al., 2002). Here, we will first focus on the factors directing cell fate determination and identity of RGCs in development, programmed RGC death during development, and factors determining their precisely ordered topographical projection to the midbrain targets. Factors supporting the maintenance of RGCs throughout normal adult life will be discussed—although these are ill-defined to date. We will then review types and mechanisms of retinal pathology, and molecular factors of RGC degeneration and apoptosis. Finally, experimental therapeutic strategies towards neuroprotection of severed RGCs will be discussed, and perspectives will be developed towards efficient promotion of survival and, eventually, axonal regeneration of injured adult RGCs.
In general, we will concentrate on aspects of visual pathway development in the chick and in rodents. Much work on retinal development and guidance molecules has been initiated in chick, and led to fundamental information on topographic order of the retino-tectal projection by tracing studies, and to the cloning of guidance molecules. Ensuing genetic work (transgenic and knockout studies) in mice has largely confirmed the results obtained in birds. Further studies, especially on lesions in the adult retina have been carried out in rats and hamsters. Therefore, the sections on degeneration, survival, and regeneration of adult RGCs will mainly consider work in rodents. Although in the past years considerable and valuable work has been conducted in fish (mainly goldfish and, more recently, genetic work in zebrafish), Drosophila, Xenopus, and C. elegans, we will largely omit work completed in these species, for sake of synthesis and homogeneity. Recently, reviews on optic system development in these species have appeared, and may complement the present paper (Cutforth and Gaul, 1997; Malicki, 1999; Mann and Holt, 2001). It is interesting, though, that many axon guidance mechanisms are highly conserved through evolution, though at a detailed mechanistic level, others may be evolutionary divergent (for review see Chisholm and Tessier-Lavigne, 1999). As a further restriction, we will concentrate on RGC intrinsic factors, and endogenous properties of this CNS neuronal population, and only touch on the cellular factors from the surrounding, i.e. astrocytes, extracellular matrix proteins/proteoglycans, oligodendrocytes, and myelin. However, these interactions will be considered to some extent in the section on regeneration. While we concentrate on a single population of CNS projection neurons, some of the principles determined in RGCs will be of more general importance for the development of CNS projections, and for maintenance, neuroprotection, and regeneration of CNS neurons.
Section snippets
Generation and cell fate specification of RGCs
The eye originates as a bilateral organ from a single field in the anterior neural plate. The primordial eye field is separated into two regions by anterior migration of diencephalic precursor cells along the midline (Varga et al., 1999). Proliferation and evagination gives rise to the optic vesicles. Their infolding into optic cups, and their progressive determination originates the optic stalk, the neural retina, and the retinal pigment epithelium.
In the retina, cell differentiation from
Outgrowth and navigation of RGC growth cones and axons
Following generation of RGCs, these neurons have to connect to their midbrain targets, in particular the SC and the LGN. RGCs generated by E16 have connected to the SC already by birth, while RGCs emerging around birth take till around P5 before they have established the projection. Thus, there is a correlation between RGC birthdate and innervation in the retino-tectal pathway (Dallimore et al., 2002). For proper midbrain targeting, RGC growth cones have to navigate within the eye towards the
Anatomy of the retino-collicular pathway
RGCs relay visual information from the retina to the primary visual centres. All RGCs send their axons to the ON; therefore, counting ON axons provides an accurate way of estimating RGC numbers. In the chick, RGC axons project to the contralateral optic tectum (Mey and Thanos, 2000; Thanos and Mey, 2001). In mammals, the primary visual centres are located in the thalamus and midbrain. The pattern of retinal projections varies considerably from species to species; in rats and mice, that
Graded expression of transcription factors
Graded expression of axon guidance molecules (see below) may be mediated by graded expression of transcription factors and other regulating proteins. Evidence for this hypothesis has been recently brought forward by the demonstration that ventroptin, a BMP-4 antagonist, is expressed in the retina in a ventral high—dorsal low gradient, and in a nasal high—temporal low gradient at later developmental stages. Misexpression of ventroptin abolished the asymmetric retinal expression of ephrin-A2 (but
Programmed cellular death and the refinement of the retino-collicular map during development
As outlined above, the early phase of retino-tectal wiring/patterning is mediated by the action of guidance molecules expressed in gradients. It is independent of electrical activity, and only attains a gross degree of precision (Nakamura and O’Leary, 1989; Simon and O’Leary, 1992). The most obvious example of topographic imprecision at these early stages is the presence of transient projections to the inappropriate side of the brain, or to topographically inappropriate regions of the primary
Molecules involved in signalling, regulation, and execution of apoptotic cell death
A body of experimental evidence in the retina indicates that RGCs die by apoptosis, and developmental cell death may be regulated by the same pathways as secondary apoptotic RGC death following ON lesions. Much insight into the process of apoptosis, and its regulation by endogenous factors has come from C. elegans. In the hermaphrodite, 131 out of 1090 somatic cells undergo apoptosis. Three molecules have been found to be key regulators of this apoptotic cell death: Ced-3 and Ced-4 promote
What keeps RGCs viable physiologically through adulthood?
In mice and rats, the development and refinement of the retino-tectal projection has reached a definite, adult state by 2 weeks of postnatal development. From this point in time, the number of RGCs and the topography of the projection including laterality persist basically unchanged throughout life. It is by no means clear, however, what determines lifelong survival of RGCs, and the stability of their projections. Two minimal requirements might have to be fulfilled for RGCs that survive into
Why do RGCs degenerate following lesion?
Due to both its unique structure, and anatomical location outside the brain, pathophysiological principles affecting RGCs can be better defined and described than in many other parts of the body, and the brain. Therefore, pathological processes involving RGCs are particularly well suited to study neurodegenerative processes, using quantitative pathological approaches. Following lesion, adult mammalian CNS neurons including RGCs degenerate, and lack the ability to regenerate (Ramon y Cajal, 1928
Neuroprotection for injured adult RGCs
Genetic models are particularly instructive to learn about the role played by single factors in development or pathology, i.e. to study biological actions. For studying therapeutic effects, they are less well suited, since therapy—in a clinical sense—can usually only be started after an insult occurred, while transgenes are expressed already before a lesion is induced. The retina offers a multitude of routes and techniques for administration of potentially protective effectors.
Regeneration in the adult CNS
Ramon y Cajal was the first to observe that transected CNS axons do not regenerate past the lesion site in adult mammals (Ramon y Cajal, 1928). This inability to regenerate is due to the combined action of two processes: a developmental loss, or reduction, of the regenerative capabilities intrinsic to CNS neurons, and the non-permissive nature of the glial scar and CNS myelin. Until few years ago it was believed that axonal regeneration of CNS neurons does not take place in the CNS of adult
Perspectives
Although our knowledge on the development, physiology, and pathology of the retino-tectal projection is ever increasing, there are still more questions unresolved than answered. While studies on single factors and particular mechanisms in development or defined pathologies will continue to yield important results, we can take it for granted that novel genomic and proteomic techniques will successfully be adapted to the retina. As one example, gene chip technology (Fambrough et al., 1999;
Acknowledgements
Our work is supported by the Deutsche Forschungsgemeinschaft (Is 64/1-2; Is 64/1-3 to S.I.).
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