MicroRNA Expression in the Glaucomatous Retina.

Glaucoma is a leading cause of irreversible worldwide blindness, characterized by progressive loss of retinal ganglion cells (RGCs) and optic nerve degeneration that is usually secondary to elevated IOP.1–3 Current treatments for glaucoma are predominantly restricted to the modulation of IOP through pharmacological and surgical approaches. However, the effectiveness of this strategy can be limited in many patients by poor medication efficacy, intolerance, or surgical failure. The primary insult leading to glaucomatous injury is thought to occur to RGC axons at the optic nerve head (ONH).4–7 In addition, patients with established glaucomatous damage may still suffer progressive loss of vision despite maximal IOP lowering. This progressive susceptibility of glaucomatous eyes to further damage may occur as a consequence of the molecular and structural mechanisms in the ONH and/or retina that occur during the disease process. An improved understanding of the biological processes in both locations may help direct future research toward the development of novel interventions to attenuate further loss of visual function in patients with glaucoma. Experimental paradigms to model glaucomatous damage have been developed in a variety of species. In models ranging from rodents to primates, chronic elevation of IOP leads to loss of RGCs in a similar manner to that observed in human glaucoma,8 with secondary cell death thought to ultimately occur due to apoptosis.9–12 Studies of gene expression within the whole retina in experimental glaucoma highlight the complex and dynamic nature of the changes that occur following the induction of glaucomatous damage, which serve to modulate both positive and negative regulatory pathways.13–19 These studies have reported common themes, including upregulation of genes associated with cell death, cell proliferation, glial activation, TNF-α signaling, immune and inflammatory responses, and downregulation of genes associated with lipid biosynthesis and cytoskeleton and light sensation.13–19 However, with the initial IOP-related insult affecting RGC axons at the ONH, the impact of the primary injury would be expected to predominantly affect RGCs within the inner ganglion cell layer of the retina. Laser capture microdissection (LCM) has been used to isolate gene expression changes within the RGC layer following induction of experimental glaucoma,16,20 allowing further refinement of both the altered functional gene classes and the magnitude of any change, by reducing the potential dilution effect from gene responses in other retinal layers. Understanding the contribution of these various responses to the development of glaucomatous damage requires a detailed understanding of the variety of signals that may further impinge on their function. MicroRNAs are small (∼18–22 nt), endogenous noncoding oligoribonucleotides that are highly conserved across species and modulate the posttranscriptional silencing of gene expression.21 They function through recognition of specific sequences in their target mRNAs and predominantly act to reduce expression of their mRNA targets.22 Understanding how these molecules modulate the mechanisms associated with glaucomatous damage may facilitate future targeting of microRNAs to attenuate glaucomatous injury. Although the influence of microRNAs on the biological processes that occur within the glaucomatous ONH and retina is not currently known, there is an emerging body of evidence in central nervous system (CNS) research that suggests they may play an important role. Experimental studies of brain and spinal cord injury have demonstrated that microRNAs influence several pathways that are also known to be modulated within the glaucomatous retina. These include apoptotic cell death,23–30 ischemia,31–39 inflammatory and immune responses,30,40–45 extracellular matrix (ECM) remodeling27,46–48 and TGF-β signaling.43,49,50 The interactions of these biological responses are highly complex. In isolation, interpretation and simplification should be performed with great caution. However, examining the pattern of microRNA expression in conjunction with gene expression changes may help in our interpretation and understanding of the mechanisms that develop following glaucomatous injury. Eyes with established severe injury are likely to demonstrate maximal responses to the ongoing primary IOP-related stimulus, in addition to the secondary mechanisms that will develop throughout the whole retina and not just restricted to the RGCs. In this study, we therefore tested the hypothesis that microRNAs altered in models of CNS injury are also altered within the glaucomatous retina with advanced damage.