Secondary degeneration after partial optic nerve transection : mechanisms and the neuroprotective effects of lycium barbarum

Abstract

Glaucoma is a neurodegenerative disease and one of the major causes of blindness in the world. Secondary degeneration is involved in glaucoma. The retinal ganglion cells (RGCs) which are vulnerable to secondary degeneration in glaucoma are the promising target population for therapeutic intervention. Partial optic nerve transection (PONT) model has been established in the last decade. Primary and secondary degeneration can be separated in different regions of retinas in this model. Therefore, PONT is a good model for the study of mechanisms of secondary degeneration and the drug screening for secondary degeneration. Lycium barbarum (L. barbarum) has been shown to be neuroprotective for cortical neurons in vitro. It has also been shown that L. barbarum could delay RGCs death in a rat ocular hypertension model. In order to further investigate the effects of L. barbarum for RGCs, two models, complete optic nerve transaction (CONT) model and PONT model, were employed in my study.

My results showed that the polysaccharide extract from L. barbarum (LBP) could partly prevent RGCs from death in the inferior retinas 4 weeks after PONT whereas it could not reduce the loss of RGCs after CONT. The1,1'-dioctadecyl-3, 3, 3’, 3’-tetramethylindocarbocyanine perchlorate(DiI) labeling of RGCs whose axons were transected showed that the majority of labeled cell bodies existed in the superior retinas. The result meant that more cell bodies in the superior retinas would die from primary degeneration than in the inferior retina after PONT. Therefore my results indicated that LBP protected RGCs which would die from secondary degeneration rather than primary degeneration.

The results of Terminal-deoxynucleoitidyl transferase mediated nick end labeling (TUNEL) staining showed that RGCs underwent apoptosis 1 week after PONT. Western-blot analysis demonstrated that oxidative stress was involved in the degeneration of RGCs after PONT. Furthermore, c-Jun N-terminal kinases (JNKs) pathway was activated which was indicated by an increase ofphospho-JNK2/3(p-JNK2/3)and phospho-c-jun(p-c-jun). Our results also revealed that orally feeding of LBP could increase the expression of manganese superoxide dismutase (MnSOD) and insulin growth factor-1 (IGF-1)and decrease the expression of p-JNK2/3 and p-c-jun.

The results from optic nerve (ON) study showed that glial cells, including astrocytes and microglias/macrophages, were activated after PONT. Oxidative stress and inflammation were involved in the process. Secondary degeneration of ON was not obvious and LBP exerted no protective effects on the survival of axons in the ON.

The multifocal electroretinography (mfERG) study showed that both the functions of inner retinas and outer retinas were damaged after PONT. The results indicated that other cell types or the synapses between different cell types were damaged in addition to RGCs. LBP could improve the function of the whole retinas, including both inner retinas and outer retinas after PONT.

In conclusion, our results indicated that LBP protected RGCs from secondary degeneration via inhibiting oxidative stress and the activation of JNK pathway.LBP could also improve the function of both inner retinas and outer retinas after PONT.