Intrinsic changes in developing retinal neurons result in regenerative failure of their axons.

Abstract

The failure of mature mammalian central nervous system axons to regenerate after transection is usually attributed to influences of the extraneuronal milieu.Using explant cocultures of retina and midbrain tectum from hamsters, we have found evidence that these influences account for failure of regrowth ofonly a small minority of retinal axons. For most of the axons, there is a programmed loss of ability to elongate in the central nervous system. We show that there is a precipitous decline in the ability of retinal axons to reinnervate tectal targets when the retina is derived from pups on or after postnatal day 2, even when the target is embryonic. By contrast, embryonic retinal axons can regrow into tectum of any age, overcoming growth-inhibiting influences of glial factors.The rate and extent of axonal growth are influenced by intrinsic properties of individual cells (1, 2) as well as by the substrate through which the axons must navigate (3). During the last decade many studies have focused on the involvement of the extraneuronal milieu in the failure of maturing central nervous system (CNS) axons to regrow over long distances. David and Aguayo (4), utilizing peripheral nerve grafts implanted in the CNS, and Schnell and Schwab (5), applying antibodies to neutralize the effects of oligodendrocyteassociated inhibitory proteins, have shown that some axotomized neurons can be induced to reextend their axons for considerable distances. Regenerating axons from the axotomized retinal ganglion cells (RGCs) extended along the entire length of peripheral nerve grafts, and some penetrated the superior colliculus (SC) up to 500 ,um from the end of the graft. A common interpretation of these findings is that, given the appropriate environment, all neurons should be able to regenerate their exons. However, despite the impressive regrowth of cut axons documented in these studies, the regenerative capacity is expressed by a limited population of neurons (5-9): many transected axons do not regrow into the peripheral nerve graft or regenerate through an area where glial cell inhibitory proteins are neutralized with antibodies.To reexamine the problem of regenerative failure, we have used the primary visual system of the developing hamster as a model, employing organotypic explant cocultures of retina and tectum. This paradigm enables independent variation of the developmental stage of each tissue and allows us to separate the contributions of source and target tissue in influencing the extent of axonal regrowth and target reinnervation.During hamster development, RGC axons leave the eye by embryonic day 10 (E10) and reach the rostral edge of the SC by E14 [day of mating = EO, and day of birth = E16 = postnatal day 0 (PO)]. The axons grow rapidly during this early stage of elongation, maintaining a simple, unbranched morphology as they invade the tectum. At about PO, they shift into a second mode of growth, referred to as the arborization mode, as they begin to emit collateral branches and to elaborate terminal ramific...

How does an axon grow?

“…Axons from P2 or older hamster retinas have lost the ability to reinnervate even embryonic tectal explants (Chen et al 1995). Purkinje cells in cerebellar slices show a similar age-related inability to re-extend axons out of cultured slices (Dusart et al 1997).…”

Purkinje Cell Survival and Axonal Regeneration Are Age Dependent: AnIn VitroStudy

“…In these two systems, identified neurons survive the axotomy caused by organotypic culture processing and are able to regenerate axonal processes in vitro and to reestablish their correct pathway and target specificity (Li et al, 1994;Chen et al, 1995;Linke et al, 1995). Moreover, the ability to regenerate is age-dependent, because the regeneration fails in cultures taken from older animals (Chen et al, 1995;Li et al, 1995). Organotypic cultures could therefore mimic the in vivo situation, and they seem particularly suitable for studying Purkinje cell survival and regeneration.…”

“…The timing of these sequential stages reveals that the end of the developmental axonal growth and regeneration-permissive critical period coincides with the initiation of Purkinje cell efferent synaptogenesis. It can be suggested that this synaptogenesis may turn off the genetic program for axonal elongation (Tetzlaff et al, 1991;Chen et al, 1995), impairing the growth of regenerating Purkinje cell axons.…”

Experimental strategies to promote axonal regeneration after traumatic central nervous system injury

“…However, besides these extrinsic in¯uences on regeneration it has become increasingly clear in the last years that intrinsic growth properties of the neurons are likely to be as important [for reviews see Fawcett (1992) and Caroni (1997)]. There is now evidence for dierences in growth potentials of various neuronal types (Rossi et al, 1995) and a decline in regenerative axonal response with neuronal age (Chen et al, 1995; 1997). While the characterization of determinants for regeneration failure is still not brought to an end, the concept emerges that axonal regeneration depends on the interplay between extrinsic cues and intrinsic properties of the lesioned neuron.…”