Ageing-Induced Decline in Primary Myeloid Cell Phagocytosis Is Unaffected by Optineurin Insufficiency

SIMPLE SUMMARY: Amyotrophic lateral sclerosis (ALS) is a devastating disease, which results in death as early as 2–5 years upon diagnosis. Finding a cure for ALS has proved to be extremely challenging due to its complex genetic background, and its still largely unknown environmental triggers. A recent study has pinpointed inefficient phagocytosis as a new common disease mechanism in patients of different genetic backgrounds. Here, we analysed if phagocytosis defects are directly linked to mutations in the OPTN gene, which encodes for optineurin protein that regulates inflammatory signalling. To this end, we used innate immune cells (macrophages and microglia) from an optineurin truncation mouse model that resembles some patient mutations. We analysed these cells at steady state and stressed by the common ALS risk factors—ageing and inflammation. We observed that ageing decreased phagocytosis, whereas inflammation decreased degradation of the phagocytosed material. However, none of these processes were affected by the OPTN mutation. This suggests that the disease mechanism in patients carrying OPTN mutations is distinct. ABSTRACT: Optineurin is a ubiquitin-binding adaptor protein involved in multiple cellular processes, including innate inflammatory signalling. Mutations in optineurin were found in amyotrophic lateral sclerosis, an adult-onset fatal neurodegenerative disease that targets motor neurons. Neurodegeneration results in generation of neuronal debris, which is primarily cleared by myeloid cells. To assess the role of optineurin in phagocytosis, we performed a flow cytometry-based phagocytic assay of apoptotic neuronal debris and E. coli bioparticles in bone marrow-derived macrophages (BMDMs), and primary neonatal microglia from wild-type (WT) and optineurin-insufficient (Optn(470T)) mice. We found no difference in phagocytosis efficiency and the accompanying cytokine secretion in WT and Optn(470T) BMDMs and microglia. This was true at both steady state and upon proinflammatory polarization with lipopolysaccharide. When we analysed the effect of ageing as a major risk factor for neurodegeneration, we found a substantial decrease in the percentage of phagocytic cells and proinflammatory cytokine secretion in BMDMs from 2-year-old mice. However, this ageing-induced phagocytic decline was unaffected by optineurin insufficiency. All together, these results indicate that ageing is the factor that perturbs normal phagocytosis and proinflammatory cytokine secretion, but that optineurin is dispensable for these processes.