Post-treatment intervention with Lycium barbarum polysaccharides rescues neurodegeneration in acute and chronic ocular hypertension rat models - a comparison with pre-treatment outcomes

Abstract

Glaucoma is a neurodegenerative disease causing retinal ganglion cell death and progressive loss of functional vision. Increased intraocular pressure (IOP) is one of the major risk factors for this multifactorial disease. While IOP lowering remains the mainstay of treatment in preventing vision loss in glaucoma patients, many undergo progressive damage despite well controlled IOP. Thus, there is an emerging concept of neuroprotection, which can potentially preserve the healthy neurons and rescue the damaged neurons by targeting disease mechanisms that are independent of IOP. In recent years, pharmacological benefits of ancient herbal medicines, Lycium barbarum fruit, have been revisited for their potential neuroprotective effects in glaucoma management. The polysaccharides extracted from the fruits of Lycium barbarum (Wolfberry) are an effective anti-aging agent that reduce oxidative stress, modulate immune response, enhance neuronal responses, and exhibit cytoprotective properties. The neuroprotective effects of Lycium barbarum Polysaccharides (LBP) have been demonstrated in neurodegenerative insults that include experimental optic neuropathies. Pre-treatment with LBP has been reported to preserve retinal ganglion cells in both IOP-dependent and -independent models of neuronal degeneration. However, there are no reports of LBP post-treatment efficacy on experimental glaucoma models. Additionally, combined in vivo structure-function tools have not been applied in previous studies to longitudinally evaluate the protective or rescue effects of LBP. Considering the possibilities of LBP to effectively translate into clinical application, this study aimed (A) To investigate and compare the efficacy of LBP post-treatment with pre-treatment application in experimental models of intraocular pressure elevation (ocular hypertension (OHT)); (B) To apply in vivo techniques for longitudinal quantification of the rate of retinal structural and functional loss or preservation under LBP treatments using in vivo optical coherence tomography (OCT) imaging and electroretinogram (ERG) measurements. Two experimental OHT models were chosen, (i) Acute ocular hypertension (AOH) rat model (80 mmHg for 120 min) and (ii) Circumlimbal suture induced chronic OHT rat model, to compare the post-treatment efficacy of LBP with pre-treatment outcomes. The longitudinal measurements of thickness of retinal layers and ERG responses were taken either at three time points (baseline, post cannulation day 10 and 28) for the AOH model or five time points (baseline, Week 4, 8, 12 and 15) for the chronic OHT study. At the end of the experimental phase, the eyes were collected for histological examination and quantification of ganglion cell layer (GCL) cell density. The findings suggested that pre-treatment with LBP delayed the onset of inner retinal layer or retinal nerve fibre layer thinning and reduced the rate of subsequent thinning in both acute and chronic OHT models. While post-treatment could not reverse the neuronal loss incurred prior to the initiation of the treatment, it reduced the rate of subsequent neuronal loss, which preserved the GCL cell density in a manner similar to that of pre-treatment use. Interestingly, post-treatment could retain both the inner (ganglion and bipolar cell) and outer (photoreceptor cell) retinal functions, which was comparable with the pre-treatment outcomes in both models of ocular hypertension. This study is the first to demonstrate the therapeutic efficacy of LBP post-treatment intervention in successfully slowing down the rate of neuronal structure and function loss in both acute and chronic OHT models. This approach has high relevance to the clinical scenario as it mimics post-treatment efficacy of neuroprotective drugs when clinical trials are undertaken. In addition, the present study also characterized the longitudinal effects of LBP over follow-up periods of 4 and 15 weeks, as compared with earlier pre-treatment reports on acute (1 week) and chronic (4 weeks) OHT models. It is worth mentioning that LBP exhibited neuroprotective and rescue effects in the chronic OHT model without altering the IOP elevation levels. Although pre-treatment use of LBP showed better neuroprotection, the beneficial effect offered by post-treatment is remarkable as compared with vehicle treatment. Therefore, LBP could be a potential treatment option for glaucoma patients who show progressive loss of vision despite maximum treatment approaches. Future studies should probe whether there are any differences in the mechanisms of neuroprotective effects between LBP pre- and post-treatment outcomes. Also, the combined efficacy of LBP along with the standard treatment of IOP lowering therapy in a preclinical setting should be evaluated in different animal models of glaucoma combined with other risk factors such as old age, systemic hypertension and diabetic mellitus.