Targeting the ER-autophagy system in the trabecular meshwork to treat glaucoma

Abstract

A major drainage network involved in aqueous humor dynamics is the conventional outflow pathway, which is gated by the trabecular meshwork (TM). The TM acts as a molecular sieve, providing resistance to aqueous outflow, which is responsible for regulating intraocular pressure (IOP). If the TM is damaged, aqueous outflow is impaired, IOP increases and glaucoma can manifest. Mutations in the MYOC gene cause hereditary primary open-angle glaucoma (POAG) by promoting the abnormal amyloidosis of the myocilin protein in the endoplasmic reticulum (ER), leading to ER stress-induced TM cell death. Myocilin accumulation is observed in approximately 70-80% of all glaucoma cases suggesting that environmental or other genetic factors may also promote myocilin toxicity. For example, simply preventing myocilin glycosylation is sufficient to promote its abnormal accretion. These myocilin amyloids are unique as there are no other known pathogenic proteins that accumulate within the ER of TM cells and cause toxicity. Moreover, this pathogenic accumulation only kills TM cells, despite expression of this protein in other cell types, suggesting that another modifier exclusive to the TM participates in the proteotoxicity of myocilin. ER autophagy (reticulophagy) is one of the pathways essential for myocilin clearance that can be impacted dramatically by aging and other environmental factors such as nutrition. This review will discuss the link between myocilin and autophagy, evaluating the role of this degradation pathway in glaucoma as well as its potential as a therapeutic target.

Keywords: Autophagy; Glaucoma; Grp94; Myocilin; Trabecular meshwork.

Figure 1. Normal intracellular processing of myocilin

After signal recognition particle (SRP) targeting and translocation of a newly forming myocilin polypeptide to the endoplasmic reticulum lumen, postranslational modifications occur to ensure proper folding of the protein. If these modifications are successful (A), the newly formed, properly folded, protein will be sent to the golgi apparatus via COPII coat formation for packaging and delivery to specific cellular organelles or the extracellular space. However, if these modifications do not properly occur (B), and the protein becomes terminally misfolded, the intra-ER degradation mechanism, known as ERAD, will degrade the protein through a proteaosomal dependent manner for peptide recycling. This method requires the ubiquitination of misfolded proteins, and is only substantial if the misfolded protein is monomeric.

Figure 2. Grp94 association with mutant myocilin promotes intracellular accumulation

Mutations in the MYOC gene lead to the production of myocilin protein that readily misfolds and is prone to aggregation. Grp94 is an ER-associated co-chaperone that recognizes pre-aggregated misfolded myocilin, preventing ERAD proteasomal degradation, leading to aggregation within the ER. Proteotoxicity caused by aggregated myocilin in the ER leads to TM cell death and aqueous outflow dysfunction, promoting the pathlogies associated with glaucoma.

Figure 3. Inhibition of Grp94 interaction with mutant myocilin promotes autophagic degradation

Through siRNA knockdown or pharmacological inhibition, Grp94 association with misfolded myocilin can be prevented, inhibiting protein aggregation. After inhibition, misfolded myocilin can be processed into transport vesicles for transport out of the ER lumen to the cell cytosol, where these vesicles fuse with lysosomes promoting autophagic degradation of the misfolded protein. As aggregated misfolded myocilin is cleared form the ER of TM cells, aqueous outflow becomes restored.