Glucose-regulated protein 94 triage of mutant myocilin through endoplasmic reticulum-associated degradation subverts a more efficient autophagic clearance mechanism.

Abstract

BACKGROUND:

Mutant myocilin accumulates in the endoplasmic reticulum for unknown reasons.

RESULTS:

Glucose-regulated protein (Grp) 94 depletion reduces mutant myocilin by engaging autophagy.

CONCLUSION:

Grp94 triages mutant myocilin through ER-associated degradation, subverting autophagy.

SIGNIFICANCE:

Treating glaucoma could be possible by inhibiting Grp94 and reducing its novel client, mutant myocilin. Clearance of misfolded proteins in the endoplasmic reticulum (ER) is traditionally handled by ER-associated degradation (ERAD), a process that requires retro-translocation and ubiquitination mediated by a luminal chaperone network. Here we investigated whether the secreted, glaucoma-associated protein myocilin was processed by this pathway. Myocilin is typically transported through the ER/Golgi network, but inherited mutations in myocilin lead to its misfolding and aggregation within trabecular meshwork cells, and ultimately, ER stress-induced cell death. Using targeted knockdown strategies, we determined that glucose-regulated protein 94 (Grp94), the ER equivalent of heat shock protein 90 (Hsp90), specifically recognizes mutant myocilin, triaging it through ERAD. The addition of mutant myocilin to the short list of Grp94 clients strengthens the hypothesis that β-strand secondary structure drives client association with Grp94. Interestingly, the ERAD pathway is incapable of efficiently handling the removal of mutant myocilin, but when Grp94 is depleted, degradation of mutant myocilin is shunted away from ERAD toward a more robust clearance pathway for aggregation-prone proteins, the autophagy system. Thus ERAD inefficiency for distinct aggregation-prone proteins can be subverted by manipulating ER chaperones, leading to more effective clearance by the autophagic/lysosomal pathway. General Hsp90 inhibitors and a selective Grp94 inhibitor also facilitate clearance of mutant myocilin, suggesting that therapeutic approaches aimed at inhibiting Grp94 could be beneficial for patients suffering from some cases of myocilin glaucoma.

• Authors: Amirthaa Suntharalingam, Jose F Abisambra, John C O'Leary 3rd, John Koren 3rd, Bo Zhang, Myung Kuk Joe, Laura J Blair, Shannon E Hill, Umesh K Jinwal, Matthew Cockman, Adam S Duerfeldt, Stanislav Tomarev, Brian S J Blagg, Raquel L Lieberman, Chad A Dickey
• Journal: The Journal of biological chemistry (J Biol Chem) Vol. 287 Issue 48 Pg. 40661-9 (Nov 23 2012) ISSN: 1083-351X [Electronic] United States
• PMID: 23035116 (Publication Type: Journal Article, Research Support, N.I.H., Extramural, Research Support, Non-U.S. Gov't)

Chemical References

• Cytoskeletal Proteins
• Eye Proteins
• Glycoproteins
• Membrane Glycoproteins
• endoplasmin
• trabecular meshwork-induced glucocorticoid response protein

Topics

• Autophagy
• Cytoskeletal Proteins (genetics, metabolism)
• Endoplasmic Reticulum (metabolism)
• Endoplasmic Reticulum-Associated Degradation
• Eye Proteins (genetics, metabolism)
• Glaucoma, Open-Angle (genetics, metabolism)
• Glycoproteins (genetics, metabolism)
• Humans
• Membrane Glycoproteins (genetics, metabolism)
• Mutation
• Protein Binding