The effects of 670nm light on retinal Müller cell gliosis following retinal stress or injury: exploring the underlying cellular mechanisms using in vivo and in vitro models

Abstract

Photobiomodulation (PBM) describes a process whereby light wavelengths of 600- 1000nm are used to initiate biological responses. PBM has been shown to attenuate inflammation and accelerate wound healing in skin and mucosal tissues. In the nervous system, it promotes recovery of injured spinal cord and optic nerve. Our laboratory has found that irradiation with 670nm light, applied prior to retinal insult, reduced photoreceptor death in retinal degeneration in vivo. However, very little attention was paid to the non-neuronal component of the retina, the macroglia of the retina.

Müller cells (MCs), the principal macroglia of the retina, are involved in supporting retinal structure and maintaining its homeostasis. MCs react to retinal stress or injuries, described as gliosis, aiming to protect neurons. However when it enters into a progressive state, it becomes detrimental to the retina. Activated MCs, if uncontrolled, release a large amount of proinflammatory cytokines and chemokines, recruiting microglias (MGs) and monocytes, which lead to further retinal inflammation. While the retinal damage is extensive, MCs undergo mitosis and thickening of their processes, which reach the subretinal space to form glial scars that inhibit nutrient delivery, leading to further neuronal death. Thus, the aim of this thesis is to investigate the effects of 670nm irradiation on activated MCs using in vivo and in vitro stress models, exploring a new avenue that may prevent irreversible retinal degeneration.

In Chapter 3, the effects of 670nm light on activated MCs using in vitro and in vivo stress models of retinal injury were investigated. Our results demonstrated that 670nm modified MC activation, both its proinflammatory and proliferative processes. This chapter additionally draws attention to the importance of appropriate timing of treatment, as there is a finite therapeutic window to effectively mitigate gliotic changes. In Chapter 4 investigated the effects of 670nm light on interaction between MCs and photoreceptors, and on subsequent MC-derived MG activation in vitro. Results confirmed that 670nm light mitigated MC gliosis induced by photo-oxidative damage (PD), subsequently reducing MG activation. This protective mechanism of action of 670nm light in MCs was associated with increased mitochondrial activity. Chapter 5 explored the cell communication between MCs and MGs in vitro, focusing on the role of exosomes. Exosomes have been discovered to carry microRNAs (miRNAs), which allows the transfer of genetic information between cells. In this chapter, IL-1β stimulation of MCs led to the release of exosomes, which stimulated MGs to upregulate expression of proinflammatory cytokines. Several miRNAs implicated in regulating inflammatory processes were identified in MC-derived exosomes in stressed MCs. Treatment with 670nm significantly reduced the expression of some of the proinflammatory genes.

The results from this thesis collectively indicate that following MC activation in retinal damage, 670nm treatment post-damage can mitigate MC gliosis and subsequently ameliorate retinal inflammation. Furthermore, this effect may be achieved through down-regulation of proinflammatory cytokine production in the retina and modification of exosome contents. Therefore, targeting activated MCs using 670nm light may be a potential therapeutic strategy in mitigating inflammation associated with the irreversible retinal degeneration. Show more