Role of Optineurin In Metabotropic Glutamate Receptor 5-Regulated Autophagy

Abstract

Autophagy is a conserved, regulated mechanism that is responsible for the degradation of misfolded proteins and recycling of cellular components. Defects in autophagy were linked to multiple pathological conditions, including neurodegenerative diseases. Metabotropic glutamate receptor 5 (mGluR5) is Gq-coupled receptor that has been shown to regulate autophagy via the mTOR/ULK1/Atg13 and GSK3β/ZBTB16/Atg14 pathways in two neurodegenerative diseases, Alzheimer’s disease and Huntington’s disease. Moreover, optineurin (OPTN), an autophagy receptor, has recently been shown to play a role in mGluR-mediated signaling but its exact role in regulating autophagy downstream of mGluR5 remains largely unknown. CRISPR/Cas9 genome editing was used to knockout OPTN in a mouse striatal cell line (STHdhQ7/Q7) and cre/lox recombination technology was used to globally knockout OPTN in C57BL/6 mice. Protein expression levels were measured in several autophagy markers, including p62, LC3β, GSK3β, ULK1, ZBTB16, and VPS34, as well as, in cell survival marker, ERK1/2 following treatment with either DHPG (mGluR1/5 agonist) or HBSS in both wild-type and OPTN knockout groups. The activation of mGluR5 resulted in an increased phosphorylation of ERK1/2 in WT cells/tissue, however had no effect on OPTN knockout groups. Furthermore, OPTN knockout groups displayed an increase in phosphorylation of ULK1-S757, LC3β and p62, and inhibition of autophagy via the ULK1/Atg13 pathway. As well, our findings show a decrease in phosphorylation of GSK3β-S9, and ZBTB16 and an increase in VPS34 levels, indicating that autophagy is inhibited via the GSK3β/ZBTB16/Atg14 pathway. Overall, these findings provide further evidence for the critical role of OPTN in mGluR5 signaling via canonical and non-canonical pathways.