Optineurin promotes aggregation of mutant huntingtin and mutant ataxin-3, and reduces cytotoxicity of aggregates

Abstract

Optineurin (OPTN), a cytoplasmic adaptor protein involved in cargo selective autophagy of bacteria, damaged mitochondria and mutant protein aggregates, is frequently seen in pathological structures containing protein aggregates, associated with several neurodegenerative diseases. However, the function of OPTN in these protein aggregates is not known. Here, we have explored the role of OPTN in mutant protein aggregation and in cytoprotection from toxicity of mutant proteins. Mutant huntingtin (mHtt) and mutant ataxin-3 (mAtax-3) showed reduced formation of aggregates in Optn-/- mouse embryonic fibroblasts as compared with wild type cells. Co-expression of OPTN enhanced aggregate formation by mHtt and mAtax-3 in Optn-/- cells. C-terminal domain of OPTN (412-577 amino acids) was necessary and sufficient to promote aggregate formation by these mutant proteins. The E478G mutant of OPTN, defective in ubiquitin-binding and autophagy, was also able to promote aggregation of mHtt and mAtax-3. OPTN and its C-terminal domain form a complex with the chaperone HSP70 known to promote mutant protein aggregation. Overexpression of mHtt or mAtax-3 induced more cell death in Optn-/- cells compared with wild type cells. Importantly, compared to wild type cells, Optn-deficient cells having mHtt or mAtax-3 aggregates showed higher level of cell death in neuronal (N2A) and non-neuronal cells. Our results show that OPTN promotes formation of mutant huntingtin and mutant ataxin-3 aggregates, and this function of OPTN might be mediated through interaction with HSP70 chaperones. Our results also show that OPTN reduces cytotoxicity caused by these mutant protein aggregates. Significance statementThe hallmark of several neurodegenerative diseases like amyotrophic lateral sclerosis, Huntingtons disease, Parkinsons disease, Alzheimers disease and Picks disease is the formation of pathological structures containing aggregated proteins, and OPTN is frequently observed in these structures. What role optineurin plays in those aggregates is not clear. Our results show that OPTN promotes aggregation of mutant huntingtin and mutant ataxin-3, and reduces cytotoxicity of aggregates in neuronal and non-neuronal cells. We suggest that OPTN provides cytoprotection in three different ways-by promoting mutant protein aggregation, by reducing cytotoxicity of aggregates and by autophagy-dependent clearance of aggregates reported earlier. These properties of OPTN provide a possible explanation for its association with various pathological structures containing protein aggregates seen in several neurodegenerative diseases.