Understanding the role of the mitochondrial AD risk gene LACTB in myeloid cells.

Abstract Background Although powerful evidence points out the importance of myeloid cells in Alzheimer’s Disease (AD), the relevance of immunometabolism requires further exploration. Our analysis integrating AD genetics and myeloid cell genomics reported that lower LACTB expression is protective for AD, and proteomics studies have confirmed it (Wingo et al., 2021). LACTB is a mitochondrial serin protein that may influence mitochondrial morphology and bioenergetics. LACTB levels are associated with succinyl‐carnitine levels (Ghazalpour et al., 2014, Suhre et al., 2011), a metabolite that has been linked to AD risk (unpublished). LACTB has also been nominated as a tumor‐related and an obesity gene, but its function is not well defined. Methods THP1 cells were treated for 6 days with small‐interfering RNA to reduce LACTB expression. iPS cells were CRISPR‐edited to obtain LACTB knock‐out (KO) lines and differentiated into microglial cells (iMGLs). Functional experiments were performed. Collected samples were outsourced for RNAseq, metabolomic and lipidomic analysis. Results we characterized, for the first time, the role of LACTB in myeloid cells. We observed that LACTB expression is increased upon differentiation (in iMGLs compared to iPSC, and in THP1 macrophages compared to monocytes) and LPS stimulation. The downregulation of LACTB in THP1 led to an increase in succinyl‐carnitine (predicted to be protective for AD), and in histone succinylation, which may modify the cell epigenetics. Transcriptomics revealed an increase in the oxidative phosphorylation, which was validated by Seahorse experiments, and in the immune response to interferon and virus. In addition, LACTB knock‐down polarized THP1 macrophages towards a DAM‐like state. Lipidomics reported significant changes in specific lipid classes and a decrease in chain saturation. These experiments will be repeated in iMGLs. Conclusion LACTB may play a role in cell differentiation and response to stimulus in myeloid cells by modifying mitochondrial respiration and lipid metabolism. As future directions, iMGLs will be xenotransplanted in WT and 5xFAD mice to study LACTB role in vivo. Unlike other AD risk genes, LACTB encodes an enzyme, reduced expression is protective, and succinyl‐carnitine can be used as a biomarker, which highlights it as a promising AD therapeutic target.