Optic nerve injury upregulates retinoic acid signaling in the adult frog visual system

Abstract

Retinoic acid (RA) is important during development, in neuronal plasticity, and also in peripheral nervous system regeneration. Here we use the frog visual system as a model to investigate the changes in RA signaling that take place after axonal injury to the central nervous system. Immunocytochemistry was used to localize different components of RA signaling within sections of the retina and optic tectum, namely, the synthetic enzyme retinaldehyde dehydrogenase (RALDH), the RA binding proteins CRABPI and II, the retinoic acid receptors RARα, β and (, and finally the catabolic enzyme CYP26A1. The levels of these proteins were quantified in extracts of retina and tectum using Western blotting. Animals were studied at 1 week, 3 weeks and 6 weeks after optic nerve transection. At the latter time point the RGC axons were re-entering the optic tectum. All the components of RA signaling were present at low to moderate levels in retinas and tecta of control, unoperated animals. In retina, soon after optic nerve injury there was a large increase in RALDH, some increase in the CRABPs, and a large increase in RGC RARβ and ( expression. These increases continued as the RGC axons were regenerating, with the addition of later RARα expression at 6 weeks. At no stage did CYP26A1 expression significantly change. In the tectum the levels of RALDH increased after axotomy and during regrowth of axons (3 weeks), then decreased at 6 weeks, at which time the levels of CYP26A1 increased. Axotomy did not cause an immediate increase in tectal RAR levels but RARα and RARβ increased after 3 weeks and RAR( only after 6 weeks. These results are consistent with RA signaling playing an important role in the survival and regeneration of frog RGCs.

Exogenous Modulation of Retinoic Acid Signaling Affects Adult RGC Survival in the Frog Visual System after Optic Nerve Injury

“…Retinoic acid (RA) is an important morphogen that controls the patterning and differentiation of neurons during development, and it has also been identified as a signaling molecule and transcriptional activator that regulates the expression of target genes, including those of growth factors and their receptors [ 35 , 36 , 55 – 58 ]. We now have similar evidence that RA and its synthetic and degradative enzymes are likewise present at low levels in the frog visual system and are upregulated after nerve injury [ 47 ]. In the present study we investigate the effects on long-term RGC survival of manipulating RA levels in the eyeball, either by applying it (or synthetic analogues thereof) exogenously, or by inhibiting endogenous RA production.…”

“…Thus, the result that disulfiram reduces RGC survival is most likely to be due to a reduction in RA synthesis, rather than inhibition of NMDA receptors. We also have immunohistochemical evidence that both the synthetic enzyme RALDH and the degradative enzyme CYP26A1 are localized together in RGCs, suggesting that RA synthesis by these neurons has an autocrine effect [ 47 ]. Our present results suggest that this autocrine effect is mediated by RARs, in particular RARβ, which are also localized in the RGCs, and are upregulated by axotomy [ 47 ].…”

“…We also have immunohistochemical evidence that both the synthetic enzyme RALDH and the degradative enzyme CYP26A1 are localized together in RGCs, suggesting that RA synthesis by these neurons has an autocrine effect [ 47 ]. Our present results suggest that this autocrine effect is mediated by RARs, in particular RARβ, which are also localized in the RGCs, and are upregulated by axotomy [ 47 ]. Activation of RARs in the RGCs would then increase their chances of survival in the face of axotomy, via distinct intracellular signaling pathways.…”

“…Both the survival rate, and regeneration speed, of the frog’s RGCs can be augmented by application of growth factors to optic nerve or eyeball, and we have studied the molecular pathways involved [ 40 , 43 – 46 ]. All the components of RA signaling are present in the frog retina and tectum, and these are upregulated after optic nerve injury [ 47 ].The purpose of the present study is to determine whether application of exogenous RA can augment long-term survival of the RGCs, whether intrinsic RA production also has an influence on survival, and to begin to elucidate the signaling pathways that are involved in these processes.…”

Regenerating reptile retinas: a comparative approach to restoring retinal ganglion cell function

“…31 Is the same true in other species? Retinoic acid signalling is also upregulated in the frog after ONI 32 and while there is normally no regeneration after ONI in mammals, a model using the neuroprotective herbal iridoid genipin does allow RGC regeneration. Inhibition of retinoic acid receptor beta expression by use of siRNA inhibits the neuritogenic actions of IPRG001, a genipin derivative.…”