Regeneration of retinal axons in the lizard Gallotia galloti is not linked to generation of new retinal ganglion cells

Abstract

Using anterograde tracing with HRPand antibodies (ABs) against neurofilaments, we show that regrowth of retinal ganglion cell (RGC) axons in the lizard Gallotia galloti commences only 2 months after optic nerve transection (ONS) and continues over at least 9 months. This is unusually long when compared to RGC axon regeneration in fish or amphibians. Following ONS, lizard RGCs up-regulate the immediate early gene C-JUN for 9 months or longer, indicating their reactive state. In keeping with the in vivo data, axon outgrowth from lizard retinal explants is increased above control levels from 6 weeks, reaches its maximum as late as 3 months, and remains elevated for at least 1 year after ONS. By means of BrdU incorporation assays and antiproliferating cell nuclear antigen immunohistochemistry, we show that the late axon outgrowth is not derived from new RGCs that might have arisen in reaction to ONS: no labeled cells were detected in lizard retinas at 0.5, 1, 1.5, 3, 6, and 12 months after ONS. Conversely, numbers of RGCs undergoing apoptosis were too low to be detectable in TUNEL assays at any time after ONS. These results demonstrate that retinal axon regeneration in G. galloti is due to axon regrowth from the resident population of RGCs, which remain in a reactive state over an extended time interval. Neurogenesis does not appear to be involved in RGC axon regrowth in G. galloti.

Expression of neuronal markers, synaptic proteins, and glutamine synthetase in the control and regenerating lizard visual system

“…The values of HuCD and Tuj1 measurements are shown in Table 2. Our previous studies revealed that transected axons begin regrowth by 2 months postlesion and they reached the tectum by 6-9 months postlesion (Lang et al, 2002). Accordingly, the Tuj1 staining in the rostral ON was usually much stronger than caudally to the lesion site at 1 and 3 months postlesion (Fig.…”

“…Next, the osteodermal plaque was put back into place and the lizards recovered and behaved normally. The postlesion timepoints for this study were selected in accordance with previous studies we did on the regenerating visual system of G. galloti (Lang et al, 1998(Lang et al, , 2002. Thus, lesioned animals were allowed to recover over periods of 1, 3, 6, 9, and 12 months.…”

“…The intrinsic properties of lizard RGCs are apparently responsible for this regenerative success. Along these lines, our previous studies revealed that damaged RGCs upregulated the immediate early gene c-jun by up to a minimum of 9 months after optic nerve (ON) axotomy, thus reflecting the prolonged time course of axonal regrowth (Lang et al, 2002). Moreover, regrowing RGC axons in Gallotia galloti were insensitive both in vitro to the nonpermissiveness of rat oligodendrocytes and myelin (Lang et al, 1998) and in vivo to other putative inhibitory microenvironmental conditions such as the persistence of reactive gliosis and the upregulation of the extracellular matrix molecules tenascin-R and chondroitin sulfate proteoglycan (Lang et al, 2002(Lang et al, , 2008.…”

“…INDEXING TERMS: retina; synaptic vesicles; axonal regrowth; axotomy; neuronal survival; white matter neurons Spontaneous regrowth of lesioned retinal ganglion cell (RGC) axons occurs in adult fish, amphibians (review in Jacobson, 1991), and certain reptiles (Beazley et al, 1998;Lang et al, 2002;Dunlop et al, 2004), but not in birds (Mey and Thanos, 1991) and mammals (VillegasPérez et al, 1988). Strikingly, retinal neurogenesis is not involved in the regenerating process in lizards, unlike frogs and fish (Beazley et al, 1998;Lang et al, 2002). The intrinsic properties of lizard RGCs are apparently responsible for this regenerative success.…”

Proliferation, neurogenesis and regeneration in the non-mammalian vertebrate brain

“…Examples of this are the growth of retina and optic tectum in amphibians and fish, and chemoreception and increase of vagal lobe in carps (Evans 1952;Brandstätter & Kotrschal 1990;Marcus et al 1999). This is further supported by the lack of, or almost diminished, adult neurogenesis in the retina and the optic tectum of reptiles and birds (Beazley et al 1998;Lang et al 2002;Kubota et al 2002). However, it is unknown whether all classes of neurons are added and integrated into the growing brain of fish, amphibians and reptiles.…”