Optineurin modulates ER stress-induced signaling pathways and cell death.

We have investigated the physiological role of the autophagy receptor Optineurin/Optn in endoplasmic reticulum (ER) stress response using cellular and animal models. In comparison to their normal counterparts, Optn-deficient mouse embryonic fibroblasts showed significantly higher cell death and caspase-3 activation upon treatment with tunicamycin and thapsigargin, inducers of ER stress. The transcript levels of some of the genes regulated by the IRE1-XBP1 and PERK-ATF4 pathways were upregulated in Optn-deficient cells, in comparison with normal cells, upon treatment with tunicamycin, and also in the brain cortex and liver of tunicamycin treated Optn-deficient mice. Also, the basal levels of IRE1α and PERK were higher in Optn-deficient cells. These results suggest that Optn modulates ER stress-induced signaling pathways and provides protection from ER stress-induced cell death.

A cytoprotective role for optineurin during mycobacterial infection of macrophages.

Autophagy has emerged as a critical innate immune mechanism for host elimination of intracellular pathogens, however, the role of the autophagy receptor Optineurin during mycobacterial infection is not fully understood. To address this lacuna, we infected bone marrow-derived macrophages (BMDMs) derived from Optn+/+ and Optn-/- mice with Mycobacterium smegmatis, and observed the infection outcome at sequential time points. While low multiplicity of infection (MOI) did not show any significant difference between BMDMs from the two groups, at high MOI Optn-/- mice-derived BMDMs showed significantly lower colony forming unit counts, as well as lower cell counts at 12 h and 24 h post-infection. Quantification of cell numbers and nuclear morphologies at various time points post-infection indicated a markedly higher cell death in the Optineurin-deficient macrophages. Optineurin-deficient BMDMs showed significantly lower levels of the autophagosomal protein LC3-II upon infection, indicating a potential role for Optineurin in regulating autophagy during mycobacterial infection. Moreover, when stimulated by bacterial LPS, Optineurin deficient macrophages, showed altered levels of the inflammatory cytokine pro-IL-1ß. These observations taken together suggest a novel regulatory role for Optineurin during mycobacterial infection. Its deficiency leads to an impairment in macrophage responses, directly impacting the pathophysiology of infection.

Identification of a splice variant of optineurin which is defective in autophagy and phosphorylation.

Optineurin (Optn) is an autophagy receptor that performs various functions in cargo-selective and non-selective autophagy. Here, we have identified and characterized a splice variant of mouse optineurin mRNA, which produces a truncated protein lacking N-terminal 157 amino acids (d157mOptn). This mRNA and protein are expressed in several tissues and cells. d157mOptn has an intact LC3-interacting region and a serine (S187) in it. However, unlike normal optineurin, the d157mOptn was not phosphorylated at this site when expressed in mammalian cells, and showed reduced interaction with TBK1 (tank binding kinase) that mediates phosphorylation at S187 (S177 in human OPTN). This phosphorylation of Optn required intact N-terminal sequence as well as functional C-terminal ubiquitin-binding domain. Unlike normal optineurin, d157mOptn was unable to promote autophagosome and autolysosome formation upon expression in Optn-deficient cells. d157mOptn was recruited to mutant huntingtin aggregates, but unlike wild type optineurin, it was unable to clear these aggregates by autophagy in neuronal NSC-34 cells. Phospho-TBK1 was seen around mutant Huntingtin aggregates in Optn overexpressing cells but it was reduced in cells overexpressing d157mOptn. Thus, we have identified an isoform of mouse optineurin which is defective in cargo-selective and non-selective autophagy possibly due to loss of phosphorylation and impaired interaction with TBK1. This isoform, which inhibits autophagosome formation in neuronal cells, might be involved in selectively modulating some of the functions of Optn, such as autophagy. Our results provide an insight into the role of N-terminal domain of Optn in various autophagic functions.