Abstract
Purpose: :
The MAP kinase Erk5 but not Erk1/2 has been shown to transmit neurotrophic signals if stimulation with neurotrophic factors occurs at the distal axon of a neuron. This suggests a role of the Erk5 pathway in neurodegenerative diseases where retrograde axonal transport is disrupted, such as optic nerve injury or glaucoma. Brain Erk5 levels in rats are known to decrease from embryonic to early postnatal age. We studied the abundance of Erk5 in retinal ganglion cells (RGCs) of young and old animals and the response of Erk5 and Erk1/2 phosphorylation after optic nerve injury and after stimulation with BDNF at the superior colliculus.
Methods: :
Immunostaining for Erk5 were performed in sprague dawley rat control retina cross-sections. Levels of total Erk5, Erk1/2 and their phosphorylated isoforms were analysed in retinal lysate of sprague dawley rats using quantitative western blot. Young (3 weeks) rats were compared with old (> 1 year) rats. Transsection of the optic nerve was carried out in 10 animals (n=4 for 1 day and n=6 for 3 days timepoint). The neurotrophic pathway of the distal RGC axon was stimulated by BDNF-injection into the superior colliculus (n=8 for treated and control 4 month old animals, 24 hours after injection).
Results: :
Erk5 is present in RGCs and retinal glia cells of young and old rats. Retinal Erk5 levels remain unchanged among age groups as do brain Erk5 levels (retinal Erk5: young 0.87+/-0.083, old 0.98+/-0.27, p=0.72; brain Erk5: young 1.11+/-0.11, old 0.89+/-0.07, p=0.14). Injection of BDNF into the superior colliculus increases Erk5 but not Erk1/2 phosphorylation (relative quantity of pErk5 and Erk1/2 in BDNF treated animals relative to control animals: pErk5: 1.09+/-0.04, p=0.05; pErk1/2: 0.88+/-0.05, p=0.03). Optic nerve transsection increased Erk5 phosphorylation after 3 days, while Erk1/2 phosphorylation remains unchanged (relative quantity of retinal pErk5/Erk5 (pErk1/2 / Erk1/2) treated vs control eye: (p)Erk5: 1.29, p=0.03; (p)Erk1/2: 1.16, p=0.25).
Conclusions: :
Age had no influence on retinal Erk5 levels. The response of Erk5 but not Erk1/2 to BDNF-stimulation at the RGC axon terminus suggests that the Erk5 pathway, analogous to previous in vitro findings (Watson 2001, Nature Neuroscience), may be important for neurotrophic signalling along the axon.
Keywords: cell survival • ganglion cells